Geology books may contain a technical terms, you are not familiar with. This cheat sheet defines some. Note, I appreciate brevity! I make words understandable, but do not necessarily dot all the i's, and may even stray a bit off the formal definitions. If you need in-depth info, buy a book. A link brings you to an in-depth text, covering or including the topic you look for.

Acicular - Formed by thin needles, often in radiating aggregates.

Adit - Horizontal entrance to a mine.

Alkaline - In chemistry referring to conditions with high pH value (i.e., above 7), but in geology and mineralogy used to indicated deficiency of silica (Quartz) and thus 'alkaline rocks' do not contain Quartz.

Amorphous - Without crystalline organisation; molecules are arranged in a more or less random pattern, and do not align themselves to a crystal lattice. Opal is the only common mineral that is at least predominantly amorphous.

Amphibole - Group of rock-forming minerals, often occurring as elongate dark crystals, intergrown in rock. See general description.

Arsenate - Mineral or compound containing the phosphate ion, AsO₄^-3. Well-known arsenates include Erythrite, Mimetite, and Zeunerite.

Arsenide - Compound containing one or more As^-2 ions. Arsenide minerals often have a metallic luster.

Basalt - Fine-grained dark volcanic rock, composed of feldspar, pyroxene, olivine, and Magnetite. Often contains voids (vesicles, vugs) carrying minerals like zeolites, Agate or Amethyst.

Bipyramid - Crystal appearing as two pyramids, joined base-to-base.

Birefringence - A substance's (in our case a mineral's) ability to split a light ray passing through into two, and displace them from the original trajectory is described as it's birefringence. This is quite large in Calcite, and easily observed in a clear piece of 'double spar', though many minerals have a less distinct birefringence.

Borosilicate - A silicate incorporating boron. The better known borosilicates include Danburite and Datolite.

Calcareous - Containing lime, often Calcite; less frequently 'appearing lime- like'.

Cleavage - Some minerals have the ability to break along distinct planes, when struck. The cleavage is often described by qualifiers ranging from 'perfect' or 'distinct' (f.ex. in Calcite and feldspars) over 'poor' or 'indistinct' (f.ex. in Quartz - no, I don't care what your book says!) to 'absent' (f.ex. in Agate). Note that 'fracture' describes the separation of the mineral in directions that do not have a cleavage - that is, a mineral may have cleavage as well as fracture - and 'parting' resembles cleavage but is separation along planes of other intergrown minerals (not twins as most mineral books claim).

Cryptocrystalline - Composed of intergrown crystals, too small to be discerned visually even by the aid of normal hand lenses or microscopes. 'Crypto' means hidden. Jasper and Agate are cryptocrystalline Quartzes.

Crystal - When used by mineral collectors, a crystal is a naturally formed object with uniform physical properties, delineated by planes (faces) intersecting at angles reflecting a regular internal structure. In a more palatable fashion: Atoms are like Lego^TM blocks, you can attach them in many ways, but invariably in some sort of regular fashion. If you keep adding Legos^TM on the sides at regular speeds (though not necessarily the same speed in every direction), you will get a monument delineated by regular, flat faces. That is a crystal to a collector. Strictly speaking, a chip of that crystal is also a crystal - it has the same properties and internal organisation - but collectors like 'idiomorphic crystals', crystals with all faces as they will develop in a void.

Crystalline - Has a regular internal organisation, even if it does not necessarily form freely developed crystals. Everything composed of crystals is called 'crystalline' and consequently most minerals are crystalline, but Agate is also crystalline, though so-called 'cryptocrystalline' because it is composed of densely intergrown needles and not of visible crystals. Every now and then rocks like granite are also called crystalline because they are an aggregate of crystalline grains (Quartz, feldspar, Epidote, mica, ...).

Crystallography - The study of crystals and their properties.

Cubic - Crystal system (please check mineralogy or crystallography book for proper definition), encompassing crystal morphologies like die (cube), tetrahedron (4 triangular faces), rhombododecahedron (12 rhomb-shaped faces), and pentagondodecahedron (12 five-sided faces). Well-known cubic minerals include Fluorite, garnets, Halite, and Pyrite.

Disulfide - Containing negatively charged groups of sulfur-bound-to-sulfur, balancing positively charged metal ions. The most important disulfide mineral is Pyrite (FeS₂)

Divalent - Carrying two electric charges, either positive (as lead ions, Pb+2) or negative (as sulfur ions, S^-2)

Druse - Hollow rock with freely developed crystals on the inside, typically as in 'Amethyst druse' - a thin shell of Amethyst crystals, formed in a basalt vesicle.

Efflorescence - The formation of crusts due to decomposition while absorbing water. Many minerals, particularly sulfates from arid climates, are efflorescent - they absorb water, 'bloom' forming crusts, and fall apart.

Element - An element is composed of atoms of one kind only. Well-known elements include iron, copper, oxygen, and sulfur. See general description.

Epithermal - See hydrothermal.

Ether - In modern chemistry a molecule composed of two organic units, tied by an oxygen atom. In classic physics (Renaissance) a medium assumed to permeate space and transmit light.

Etymology - The origin and derivation of words.

Exoskeleton - External skeleton like a shell; an external hardened substance, supporting and protecting the body of an organism.

Feldspar - Group of rock-forming minerals with distinct cleavages. See general description.

Fluorescence - Some compounds, including some minerals, emit a visible light when irradiated by (invisible) UV light; that phenomenon is called fluorescence. Some minerals are fluorescent (f.ex. Autunite, Scheelite, and Tugtupite) by themselves, others (f.ex. Calcite) may be fluorescent if they contain traces of an activator (traces of manganese may act as an activator and cause superb red fluorescence in Calcite).

Fluoro-aluminate - Containing negatively charged complexes of aluminum fluoride, balanced by positive ions (in minerals usually sodium, calcium, or potassium). The best known fluoro-aluminate mineral is Cryolite, that occurred abundantly in Ivigtut (Greenland, Denmark).

Fossil - Used to mean 'something found in the earth' and until around 1800 would have been used to describe any of the things we today characterise as minerals, rocks, artifacts (if found in the earth), or fossils. Today it is used exclusively for remains of organisms of appreciable age (usually one million years or more, by arbitrary definition), that may or may not have been altered into stone (petrified).

Gangue - Traditional term, describing minerals associating ore, but not carrying metals of value. The more abundant of these include Barite, Calcite, Dolomite, Fluorite, and Quartz.

Gem - Minerals, cut and polished for jewellery are called 'gems' though the term is generally reserved for fairly hard and valuable materials like Diamond, Emerald, Ruby, and Sapphire.

Geode - Rock formed inside vesicle, typically like Agate geodes that fill voids/vesicles in basalt.

Geology - Earth science, the study of the Earth, its composition and processes.

Gneiss - Igneous rock, chiefly composed of intergrown grains of Quartz and feldspar, where Quartz dominates over feldspar. Grain size usually fairly uniform in a given formation, but may vary considerably between different gneisses. See granite as well.

Granite - Igneous rock, chiefly composed of intergrown grains of Quartz and feldspar, where the feldspar dominates over Quartz, but Epidote, Fluorite, and Zircon may be locally abundant. Grain size usually fairly uniform in a given formation, but may vary considerably between different granites. See gneiss as well.

Gravel - Unconsolidated pieces of rock, sized 2 mm - 2 cm across, usually formed by weathering of solid rock.

Hexagonal - Crystal system (please check mineralogy or crystallography book for proper definition), encompassing usually slender crystal morphologies with a six-fold symmetry. Well-known hexagonal minerals include Apatite, Beryl, Mimetite, and Vanadinite.

Hydrated - Containing water. In mineralogy and chemistry usually in the context of 'hydrated compounds' meaning something containing a specific number of water molecules as part of the crystal lattice. Gypsum, CaSO₄×2H₂O, is hydrated calcium sulfate, containing two units of water for each unit of calcium (Ca).

Hydrothermal - Often in the context of 'hydrothermal deposit' or 'hydrothermal vein', describing deposits or veins formed by aqueous solutions of minerals at elevated temperatures - often in the range 40-125 °C but may be as little as a few degrees above zero. Sometimes separated into 'hypothermal', 'mesothermal', and 'epithermal' distinguishing high, intermediate, and low temperatures. Note most deposits are formed over a range of temperatures, varying during the period of formation.

Hydroxide - Compound containing negative OH- ions balancing positive metal ions. There are only very few straight hydroxide minerals, the better known are Gibbsite, Al(OH)₃, and Brucite, Mg(OH)₂, but many minerals contain hydroxide ions as well as others. Some of these, particularly when similar minerals incorporating fluorine (F) or chlorine (Cl) in stead of hydroxide exist, are designated by a 'Hydoxy' or 'Hydroxyl' preceding the mineral name. Hydroxylapatite is an example of this.

Hypothermal - See hydrothermal.

Inclusion - Something imbedded in something else. Particularly in gemology, inclusions - microscopic grains of minerals imbedded in gems - are important to identify whether gems are natural, and where they come from.

Intrusion - Some rocks or complexes of rocks form 'intrusions', huge bodies that interject previously deposited rocks. Note the difference between 'veins' that are rarely more than a few meter across and are deposited in cracks, usually formed by shrinking of the existing host rock (f.ex. due to cooling), and 'intrusions'Cheateters across and displace existing rock due to energy stored in the intruding rock.

Invertebrate - Outdated term, albeit still much in use, for organisms without an internal skeleton (vertebrae and other bones). Mollusks, insects, worms, etc. are invertebrates.

Isometric - Alternative name for the cubic crystal system.

Lapidary - Strictly speaking 'concerned with stone' [The Concise Oxford Dictionary], but colloquially often used in the meaning 'suitable for producing cut and polished stones.' 'Lapidary machines' will include saws with Diamond blades, grind stones, etc., and a 'lapidary material' will be a stone suitable for cutting.

Lattice - Crystal lattice, the regular organisation of chemical elements into a three-dimensional grid is called the 'crystal lattice' of a compound.

Limestone - Rock composed chiefly of Calcite, Dolomite or rarely Aragonite; chiefly calcium carbonates or calcium-magnesium carbonates, though often with interesting associate minerals.

Luster - The reflection of light from the surface is called luster. It is often described by a qualifier like 'metallic luster' or 'silky luster'.

Malleable - Can be shaped by beating without breaking. Most metals are malleable, they change shape when you beat them with a hammer.

Mesothermal - See hydrothermal.

Metamorphic - Rocks that are formed by alteration due to elevated temperature and pressure are called 'metamorphic' (i.e., transformed). The alteration can either be due to compression during folding of mountains (very common in the Alps) or intrusion of hot magmas, forming a metamorphic zone around themselves in the host rock.

Micrometer - One millionth of a meter, a thousandth of a millimeter. It is designated by the symbol µm.

Mineral - See general description.

Mollusk - Organism belonging to a group including gastropods, bivalves, cephalopods, and a few smaller and lesser known groups. Many mollusks form shells composed of Aragonite or Calcite ('sea shells'). Corals, sea urchins, star fish, etc. are not mollusks.

Monoclinic - Crystal system (please check mineralogy or crystallography book for proper definition), encompassing mainly elongate crystal morphologies with a rhombic cross-section. Well-known monoclinic minerals include Antimonite, Arsenopyrite, Azurite, and Malachite.

Nacre - Nacre is a shell structures found in some mollusks, composed of Aragonite polygonal to rounded tablets arranged in broad, regularly formed, parallel sheets.

Nacreous - Containing nacre.

Nucleation - The first step of crystal formation is called 'nucleation', that is forming a 'nucleus' or core. Precisely what goes on is not necessarily well understood, nor are necessarily the conditions causing formation of a crystal in one place rather than another. Whatever the process, the first steps leading to the formation of a crystal is called 'nucleation'.

Ore - Mineral or rock from which metals or other valuable substances are extracted.

Orthorhombic - Crystal system (please check mineralogy or crystallography book for proper definition), encompassing mostly blocky crystal morphologies with a at least three mirror planes. Well-known orthorhombic minerals include Andalusite, Barite, Celestite, and Topaz.

Oxide - Compound containing negatively charged oxygen ions, balancing positively charged metal ions. To the unwitting observer, this seems to include most known minerals, but mostly the oxygen is bound more firmly to 'something else' than to the metals, forming 'functional groups' like sulfate (SO₄^-2), arsenate (AsO₄^{- 3}), nitrate (NO₃-), or silicate (f.ex. Si₂O₇-6), and these 'functional groups' are rarely decomposed when the minerals dissolve.

Pantheon - Best preserved building of ancient Rome, retaining a huge dome made from cement [the recipe was subsequently lost and not rediscovered until the 18th century]. Built by Marcus Agrippa in 27 BC, heavily rebuilt by Hadrian 120, and plundered by the usual gang of emperors and popes since then.

Parting - Some minerals exhibit what appears to be a distinct cleavage, but it isn't! It is 'parting' and the better known minerals with prominent parting are Corundum, Cryolite, and Hematite. Many reference books erroneously ascribe parting to the separation of twins, but in carefully investigated samples (e.g., Corundum and Hematite) it is a separation of the mineral in question and thin films of other minerals (f.ex. Bšhmite), selectively growing parallel to certain faces in the crystal. That is, parting is not a property of f.ex. Corundum, but a consequence of thin blades of Bšhmite growing selectively on faces of the crystal.

Pegmatite - Relatively coarse-grained rock in vein, that is rarely more than a few meters wide. Formed from the residual fluid from intrusions, deposited as veins in cracks formed by the shrinking of that same intrusion. Composition often 'granitic' (coarse grains of feldspar and Quartz), but usually carries exotic minerals as well.

Pentagondodecahedron - Crystal shape of twelve five sided faces, belonging to the cubic crystal system. Pentagondodecahedra are fairly common in Pyrite and a few other Pyrite-group minerals, but otherwise quite unusual.

Phosphate - Mineral or compound containing the phosphate ion, PO₄^-3. Well-known phosphates include Apatite, Pyromorphite, and Turquoise.

Pliny - Plinius Secundus (23-79), Pliny the Elder, Roman geezer who wrote a 'Natural History' in 37 volumes, compiling observations and previous knowledge on natural history, including geology. He is often our best reference to antique science and superstition and ignorance, all at the same time! Plinius died in Pompeii, watching the eruption of Vesuvius in 79.

Primary - Minerals are often characterised, somewhat arbitrarily as 'primary' and 'secondary', where the primary were deposited by whichever geologic process (f.ex. sulfide ores in a hydrothermal vein), and secondary formed by alteration of the primary (f.ex. Azurite in the oxidation zone). Note, secondary minerals may also include f.ex. sulfides in the cementation zone; they are not invariably oxidised.

Pseudomorph - Minerals adopting the external morphology of others, while retaining their own internal organisation are called 'pseudomorphs'. See general description..

Pyroxene - Group of rock-forming minerals, often occurring as elongate dark crystals, intergrown in rock. See general description.

Quarry - Open hole in the ground from which minerals or ore is extracted. Note, a quarry is really an open hole in the surface of the earth, and consequently generally of restricted depth, contrary to a mine, that may reach considerable depth but are operated through a small opening at the surface.

Radioactive - See general description.

Radioactivity - See general description.

Renaissance - A period of revival of arts and sciences in the 14th-16th centuries Europe under the influence of classic ideals. The principle of observational and interpretative science (contrary to the antique philosophical science) was founded in the Renaissance.

Rhombic - Different name for orthorhombic crystal system.

Rhombohedron - A solid bounded by six rhombs ('diamonds'). Cubes and 'bricks' are special rhombohedra with right angles.

Rock - Hard throwable object. In geology used for an aggregate of minerals, often forming mountains. Well-known rocks include granite, basalt, and limestone.

Schist - Coarse grained rock with distinct layering, defined by platy or elongate minerals.

Secondary - See primary.

Sediment - Rock formed by sedimentation, the gradual accumulation of smaller grains. The grains can be relatively coarse (like sand in sandstone) or fine (like clay), formed by decomposition of other rocks and brought to the sea by rivers, and deposited at sea, or formed by microscopic organisms like coccoliths (calcareous algae) forming small pieces of calcite, deposited at the bottom of the sea and forming limestone.

Sedimentary - Processes forming sediments.

Serpentine - A group of magnesium silicates, often formed by the alteration of Olivine or pyroxene. Serpentine minerals often associate interesting, collectible minerals, but can be quite difficult to identify themselves.

Slag - Whatever remains of rock after smelting out the metals is called 'slag'.

Spar - Several classic mineral names contain 'spar' referring to their ability to form distinct cleavages. Some of the better known are calc spar (Calcite), fluor spar (Fluorite), and obviously feldspar. The systematization of mineral names has largely done away with names ending on 'spar', etc. replacing with 'proper scientific names' ending on -it [-ite is a phonetic abomination in English, not required by science!].

Species - In minerals, whatever we decide it to be! Mineral species are approved or rejected by the International Mineralogical Association, considering data submitted by researchers. The criteria for accepting a new species as valid change over time (we learn and change our ways - not a bad approach to science), but are not arbitrary even if not rigidly defined.

Submetallic - Submetallic luster is a bit less lustrous than metallic (we are in the territory of poor definitions!). Of common minerals, 'dull' Magnetite or Hematite have submetallic luster, though 'bright' Magnetite or Hematite have metallic luster.

Sulfide - Contains negatively charged sulfur ions, balancing positive metal ions (at least mostly metals). Common sulfide minerals include Galena (lead sulfide) and Sphalerite (zinc sulfide).

Sulfosalt - A traditional term decribing minerals with a combination of sulfide and either of antimony or arsenic as negative ions balancing the positive metals. Many of these are inconspicuous grey to black submetallic to metallic minerals, but a few like Proustite and Pyrargyrite produce very desirable and highly valuable collectors items.

Syenite - Unusual type of coarse grained rock, poor in Quartz. So-called alkaline syenites like Nepheline syenite lack Quartz, and often carry a host of otherwise rare minerals. Huge alkaline complexes like Chibina and Lovozero in Kola (Russia), Langesundsfjord (Norway), and Ilimaussaq (Greenland) are dominated by syenites and rare minerals.

Synthetic - Produced by man in a laboratory or factory. Often used in conjunction with a mineral name; 'synthetic Diamond' is cubic carbon, with the same crystal structure as the mineral Diamond, but produced in a factory - it is often used industrially as an abrasive.

Tabular - Plate-like, flat.

Tetragonal - Crystal system (please check mineralogy or crystallography book for proper definition), encompassing usually slender crystal morphologies with a four-fold symmetry. Well-known tetragonal minerals include Apophyllite, Cassiterite, Rutile, and Zircon.

Theophrastus - Greek fellow who around 315 BC wrote 'On Stones', of which only part is preserved. One of or earliest references to antique (European) knowledge on and usage of stones. Interesting to read, but not the first place you will go for exact information on minerals.

Translucent - Light may pass through, but you can not see through it. A normal piece of paper is translucent - you can see light through it and may distinguish shades, but you can not see through the paper and form an image of what is on the opposite side.

Transparent - Light passes through, and you can see through it; you can actually form an image of what is on the opposite side. Most glass is transparent. Note that transparency does not reveal anything about colour - green glass and white glass are both transparent; one has colour, the other does not.

Triclinic - Crystal system (please check mineralogy or crystallography book for proper definition), encompassing crystal morphologies with no symmetry elements (mirror planes, rotation axes). Well-known triclinic minerals include Axinite, Plagioclase (a group of feldspars), and Rhodonite.

Trigonal - Crystal system (please check mineralogy or crystallography book for proper definition), encompassing usually slender crystal morphologies with a three-fold symmetry. Well-known trigonal minerals include Calcite, Dioptase, and tourmalines.

Trilling - Three individual crystals, intergrown in a regular fashion around mirror planes are called a trilling (and not a triplet!).

Type locality - The locality, where the type specimen was found. The type locality will often contain a fair number of specimens from the same place and with the same appearance as the type specimen, and it is interesting to form a collection of minerals from their respective type localities.

Type specimen - When a mineral is described, it is always based on a particular specimen. That individual specimen is called the 'type specimen' and museums are always quite protective of their type specimens. If the original analysis of the type specimen is ever questioned, the definition of a mineral species is questioned, and it must be possible to go back and verify and maybe correct the analysis. That is only possible if the specimen exist and is stored in a safe place.

Twin - Two individual crystals, intergrown in a regular fashion around a mirror plane is called a twin.

Ultraviolet light - See UV-light.

UV-light - Light with wavelengths 0.6-380 nm is called ultraviolet or UV light. It has more energy per photon and has shorter wavelength than visible light. UV light is often used by mineralogists to induce fluorescence - UV light reaching a mineral may add energy, some of which is returned in the form of visible light, fluorescence.

Variety - The definition of mineral species is at best vague, and by consequence the definition of variety must be even more vague. Nevertheless we often use the term variety, often in the form of f.ex. 'Adamite var. Cu-Adamite'. Some specimens have distinct properties, setting them apart from the standard - be it by incorporating an appreciable amount of a chemical element, a distinct colour or habit - but not enough to warrant acceptance as an individual species. Such specimens are often designated as a 'variety'. Adamite is a zinc hydroxyl- arsenate, and Olivenite the corresponding copper hydroxyl-arsenate, and the two minerals are miscible - that is, a given specimen can have any ratio of zinc/copper. Most specimens are, however, close to either end, but those that are not - Adamite specimens with appreciable copper and Olivenite specimens with appreciable zinc - can be identified as 'Adamite var. Cu-Adamite' or 'Olivenite var. Zn-Olivenite' respectively.

Vein - Many minerals originate in veins, f.ex. hydrothermal veins or pegmatite veins. These veins are not elongate, rod-like strings like blood veins, but more like flat, irregular plates. They form in cracks in the host rock, and fluids penetrate the cracks and deposit minerals in them.

Vertebrate - Outdated term, albeit still much in use, for organisms with an internal skeleton (vertebrae and other bones). Dogs, horses, fish, etc. are vertebrates.

Vug - A cavity in rock is often called a vug, and crystals may grow on the inside. Note, that the crystals grow in a pre-existing hole, they do not form or expand the hole.

Zeolite - Group of minerals often found in basalt vugs. See full description of group.