Corundum
Few minerals are subject to as much lore and as many myths as Corundum, chiefly in
the form of the gems Ruby and Sapphire. This can only be a very brief introduction, and
we warmly recommend Richard Hughes' monumental work 'Ruby and Sapphire' as an
authoritative guide to valuable information, not just the usual rehashed hogwash of
popular guides. This book also has an extensive bibliography, you may wish to consult
for additional information. Apart from being a gem, Corundum is used industrially as
an abrasive, known as emery. Corundum is considerably softer than Diamond, but still
much harder than most other substances, and far more economical in use - emery is
found in huge deposits, and is easily extracted.
Corundum forms a minor component of a number of metamorphic rocks, marbles
and schists, but also of granite, pegmatite, syenites, and others. It is often unevenly
distributed in the rock, and you observe rich pods with a high volume Corundum in an
otherwise nearly barren rock.
Ruby and Sapphire are some of the most cherished and valued of all gems. They
are both colour varieties of the mineral Corundum, but there is no clear definition of
them. Corundum can take almost any colour, and most of natural Corundum is either
grey, brown or black, and unsuited for gems. In the old days, more or less every red
Corundum would be a Ruby, every blue was a Sapphire, and all the other colours were
troublesome! In Antiquity through the Renaissance it was common to characterise
any blue gem, including particularly Lapis Lazuli, as 'Sapphire' and any red gem as
'Ruby'. The Black Prince's Ruby in the British crown is, for example, a red
Spinel.
More modern terminology requires Sapphire to be 'corn flower blue' Corundum,
Ruby to be 'red like fresh pigeon blood', and all other colours (yellow, pink, peach, ..)
are 'fancy Sapphires'. It sounds very nice, but is not very practical. Imagine going to the
jeweler to pick a ruby. Not only do you have to carry a live pigeon with you, you also
have to chop its head off in the shop and compare the stones to the colour of the blood.
Generally, jewelers don't like that! For practical purposes, it is better to assume
rubies are dark, clean red - not purplish, not brownish, not pink - and sapphires are
clean, dark blue. Everything else is a 'fancy sapphire'.
Corundum can be found in several different types of 'silica deficient' rock, that is
rocks without Quartz. These include syenites with much feldspar and amphiboles,
limestones, as well as a few more exotic types. Corundum is very hard, and it is often
mined from so-called secondary deposits like river gravels, where the Corundum is
preserved even if the rest of the rock has decomposed. As a matter of fact, only very
little Corundum is mined from primary rocks.
Most of the great classic deposits of Ruby are in Asia. The most important are
Mogok in Mandalay, Myanmar (Burma) and Jagdalak, 50 km E of Kabul in Afghanistan.
Both produce beautiful rubies from a white limestone and nearby river gravels. Ruby
also comes from Mysore in Madras, India and Ratnapura and Rakwana in Sri Lanka
(Ceylon), as well as a few places in Africa. A very attractive material, bright red Ruby
in deep green Zoisite comes from Longido in the Matubitu Mountains, Tanzania. This is
rarely of gem quality, but a very attractive specimen material.
There are far more deposits of good Sapphire, though most of these are also
Asian. By far the best and most cherished of a natural, deep blue colour, good size and
superb translucence come from 4500 meter elevation in Soomjam, Padar, in the Indian
part of Kashmir. Good, large stones are very rare, but you may have seen one of them
more than once: Diana Spencer got an important Kashmir sapphire in her engagement
ring from her future husband, the Prince of Wales. Far more productive deposits are
situated in Khao Ploi Waen, Bo I Ram, and Bang Ka Cha in Eastern Thailand, Ratnapura
and Rakwana in Sri Lanka (Ceylon), Battambang in Cambodia, but also in Africa.
Recently there have been a virtual 'gold rush' for gem Sapphire in Madagascar, and
'hotel' rooms (bug infested mattresses, dirty linen, no bath, no breakfast) in a distant
rural area suddenly rose from next to nothing to US $ 100 per night!
Ruby and Sapphire may have a six-sided star, that is revealed by cutting, giving
attractive dome-shaped stones, with a silky star on top. The start is due to fine needle
inclusions, often of the mineral Rutile. Inclusions are very common in Corundum, only
very few are naturally transparent, but with heat treatment it is possible to dissolve the
inclusions, leaving a transparent stone. Heat treatment is also used to change colour of
Corundum. Virtually all Thai Corundum is brown, when collected, but turns dark blue
when heated. Many pale rubies with inclusions turn dark red and transparent when
heated. This is not 'cheating', as long as the stone is not sold under the pretext of being
untreated, just as cutting it is not 'cheating' - that too is a profound alteration of the
stone's natural properties.
Specimen Handling
Corundum is stable in a normal household environment - it is one of the most stable and
inert minerals known. It is not harmed by light, changes in temperature in the normal
comfort range, or known to decompose. Corundum specimens can be fragile and should
be handled with care like any other mineral specimen. Corundum is not appreciably
soluble in water.
Bibliography
Arrouas, S. 1993. Rubis du Vietnam, mythe ou realite. Revue de Gemmologie, 115, 7-
8.
Bancroft, Peter. 1984. Gem & Crystal Treasures.
Baric, Ljudevit. 1977. Im weissen Marmor. Korunde aus Mazedonien. Lapis 2(8),
10-12
Bauer, Max. 1896. Edelsteinkunde, 1st ed.
Bauer, Max. 1909. Edelsteinkunde, 2nd ed.
Bauer, Max. 1968. Precious Stones, I-II. Dover Publications
Bernard, A. 1975. Les saphirs de Houei Sai. Bulletin - Association Francaise de
Gemmologie, 43: 9 pp.
Berrange, Jevan P., Jobbins, E. A. 1976. The geology, gemmology, mining
methods and economic potential of the Pailin ruby and sapphire gem-field, Khmer
Republic.
Brown, J. Coggin. 1927. Gem mining in the Mogok stone tract of Upper Burma
from the annexation to the present (with some suggestions for future operations).
Confidential report, recirculated in J.Coggin Brown, 1936, India's mineral
wealth. London, 345 pp.
Chhibber, Harbans Lal. 1934. The mineral resources of Burma.
Colombo, Marco, Antonello Donini, Elena Gambini, Daniella Muzzioli, Giuliano
Radice & Margherita Superchi. (eds.) 1996. Gemmologia Europa V, I
gemmologi europei raccontano rubini e zaffiri, European gemmologists on rubies
and sapphires. Gemmologia Europa V.
Dele-Dubois, M. L., et al. 1993. Rubis du Vietnam, Etude comparative avec les
rubis de Birmanie et d'autres provenances. Revue de Gemmologie, 114, 7-10.
Gaines, Richard W., H. Catherine W. Skinner, Eugene E. Foord, Brian Mason,
Abraham Rosenzweig & Vandall T. King. 1997. Dana's new mineralogy:
the system of mineralogy of James Dwight Dana and Edward Salisbury Dana, 8th
ed.
Gübelin, E. J., Peretti, A. 1997. Sapphires from the Andranondambo Mine
in SE Madagascar, evidence for metasomatic skarn formation. Journal of
Gemmology, 25(7), 453-470.
Gübelin, Eduard, Max Weibel, Kamal Girgis, Roland Wessicken. 1977. Die
Struktur der Sterne. Lapis 2(8), 19-26.
Gübelin, Eduard. 1977. Im Tal der Rubine. Die Rubin-Lagerstätten
von Mogok in Birma. Lapis 2(8), 19-26.
Gübelin, Eduard. 1979. Fiskenässet. Rubin-Vorkommen auf
Grönland. Lapis 4(3), 19-26.
Gübelin, Eduard. 1982. Die Edelseinvorkommens Pakistans. I. Die Rubine
aus dem Hunzatal. Lapis 7(5), 19-26.
Heinz Meixner. 1977. Rubin von Longido, Tansania. Lapis 2(8), 13.
Henn, U., et al. 1990. Red and orange corundum (ruby and padparadscha) from
Malawi. Journal of Gemmology, 22(2), 83-89.
Henn, Ulrich. 1991. Burma-type rubies from Vietnam. The Australian
Gemmologist, 17(12), 505-509.
Hintze, Carl (ed.) 1904-1915. Handbuch der Mineralogie, vol. 1, section 2.
Holland, Thomas Henry. 1898. A Manual of the Geology of India. Economic
Geology. Part 1, Corundum. Calcutta.
Hughes, Richard. 1997. Ruby & Sapphire.
Jobbins, E. A. & Berrange, J. P. 1981. The Pailin ruby and sapphire
gemfield, Cambodia. Journal of Gemmology, 17(8), 555-567.
Kammerling, Robert C., et al. 1994. Update on mining rubies and fancy sapphires
in northern Vietnam. Gems and Gemology, 30(2), 109-114.
Kane, Robert E., et al. Rubies and fancy sapphires from Vietnam. 1991. Gems
and Gemology, 27, 136-155.
Keller, Peter C. 1992. Gemstones of East Africa.
King, Francis P. A Preliminary Report on the Corundum Deposits of Georgia.
Geol. Survey Georgia, Bull. #2, 1894. Quarto, pp. 138, colored map, 6 pl. text
figures.
Kock, W.P. de. n.d. (1930s) Corundum prospect on the farm Hochfeld 112
Warmbad District.
Kong Bei, Zou Jinfu, Yongquan Luo & Shijie Huang.1999. Coloration
mechanism and heat treatment of Betsiaka Sapphire in Madagascar. Guilin Gong
Xueyuan Xuebao = Journal of Guilin Institute of Technology, 19(2), 123-125.
Links, G.F. 1978. The Chester Emery Mines. Mineralogical Record, 9, 235-
242.
Noe-Nygaard, Arne. 1966. Mineralogi, 3rd ed.
Palache, Charles, Harry Berman & Clifford Frondel. 1944. The system of
mineralogy of James Dwight Dana and Edward Salisbury Dana, Yale University
1837-1892, 7th ed. Vol. 1.
Ramdohr, Paul & Hugo Strunz. 1980. Klockmann's Lehrbuch der
Mineralogie, 16th ed.
Roberts, Willard Lincoln, Thomas J. Campbell & George Robert Rapp jr.
1990. Encyclopedia of Minerals 2nd ed.
Rutland, E. H. 1963. Corundum and amethyst from Tanganyika. Journal of
Gemmology, 9(4), 132-13.
Sasum, U., et al. 1996. Spectroscopic investigations on natural rubies and
sapphires from Vietnam. 1996. Terra Abstracts, 8, Suppl. 1, 57.
Schlüssel, Roland. 2002. Mogok - Myanmar. Eine Reise durch Burma zu
den schönsten Rubinen und Safiren der Welt. Christian Weise Verlag,
München.
Schmetzer, Karl. 1977. Chrom, Eisen & Titan. Ursachen für die
Farben von Rubin und Saphir. Lapis 2(8), 8-9.
Schwarz, Dietmar, et al. 1996. Sapphires from the Andranondambo region,
Madagascar. Gems and Gemology, 32(2), 80-99.
Sinkankas, John. 1964. Mineralogy.
Steele, I.M., F.C. Bishop, J.V. Smith & B.F. Windley. 1977. The
Fiskenæsset complex, West Greenland, part III. Grønlands Geologiske
Undersøgelse, Bulletin. 124.
Sutherland, F.L., D. Schwarz, E.A. Jobbins, R.R. Coenraads, & G. Webb.
1998. Distinctive gem corundum suites from discrete basalt fields, a comparative
study of Barrington, Australia, and West Pailin, Cambodia, gemfields. Journal of
Gemmology, 26(2), 65-85.
Themelis, Ted. 1992. The heat treatment of ruby and sapphire.
Wild, Georg. O., et al. 1947. Absorption of sapphire in the ultra-violet region and
investigations on Madagascar amethyst. Gemmologist, 16, no. 195, 279-281.
Windley, B. F. & J.V. Smith. 1973. The Fiskenæsset Complex, West
Greenland. - Part II. Meddelelser om Grønland, 196(4).
Windley, B. F., R. K. Herd & A. A.Bowden. 1973. The Fiskenæsset
Complex, West Greenland. - Part I. Meddelelser om Grønland, 196(2).
This page is written and maintaned by Claus
Hedegaard
