History of Ruby & Sapphire Heat Treatment

1 July 1995
By Richard Hughes
History of Ruby & Sapphire Heat Treatment

A brief history of the heat treatment of ruby & sapphire, from ancient times to the modern era.

 A Brief History of Heat: Heat Treatment of Ruby & Sapphire

The history of gem treatments is as old as the gem trade itself. Like much gemological, one of the earliest references to gem treatments is found in Pliny's History of the World. The following is a selection:

…Moreover, I have in my library certain books by authors now living, whom I would under no circumstances name, wherein there are descriptions as to how to give the color of smaragdus [emerald] to crystallus [rock crystal] and how to imitate other transparent gems: for example, how to make a sardonychus [sardonyx] from a sarda [carnelian, in part sard]: in a word to transform one stone into another. To tell the truth, there is no fraud or deceit in the world which yields greater gain and profit than that of counterfeiting gems.

Pliny [23–79 AD], from Ball, 1950, p. 195

Another early work mentioning gem treatments is that of an anonymous Egyptian whose writings have survived in the form of two papyri believed to date from the third or fourth centuries AD (Nassau, 1984). The second, known as the Stockholm Papyrus or Papyrus Graecus Holmiensis, details methods of counterfeiting and treating gems. Below is but a small sample:

19. Production of Ruby
The treating of crystal [rock crystal] so that it appears like ruby. Take smoky crystal and make the ordinary stone from it: Take and heat it gradually in the dark; and indeed until it appears to you to have the heat within it. Heat it once more in gold-founder's waste. Take and dip the stone in cedar oil mixed with natural sulphur and leave it in the dye, for the purpose of absorption, until morning.
53. Corroding and Opening Up of Stones
Grind alum and melt it carefully in vinegar. Put the stones therein, boil it up, and leave them there over night. Rinse them off, however, on the following day and color them as you wish by use of the recipes for coloring.

ER Caley, 1927, The Stockholm Papyrus

Heat treatment of corundums has been mentioned in a number of early works, particularly by Arabs. Teifaschi, in his treatise on gems from about 1240 AD, had this to say:

In Sarandib [Sri Lanka] and its environs, ruby is treated by fire. People take pebbles from the earth and crush and compress them into a mass with the aid of water. [This mixture] is daubed completely around a dry stone. Then, the whole thing is placed on a rock with other rocks set down around it. Dry firewood is thrown on top, lit and blown upon [with bellows]. The blowing is applied, along with more wood, till any black overtones on the ruby have disappeared.

The amount of fire and the application of wood depends on the extent of the blackness present. People know this by experience. They heat-treat stones for at least one hour and, at most, twenty days and nights. Then, they carefully extract the ruby, its blackness having disappeared.

The ruby is not heat treated a second time. After one treatment, its color can neither improve nor diminish.

Teifaschi, ca. 1240 AD,
into French by Clément-Mullet (n.d., ca. 1982)
into English by Sersen (1991)

In the sixth book of his Natural Magick, John Baptist Porta [ca. 1535–1615] of Naples, discusses various methods of counterfeiting and adulterating precious stones. Most interesting is the following description of heat treatment:

How to make a stone white on one side, and red or blew on the other.
I have seen precious stones thus made, and in great esteem with great persons, being of two colours; on one side a Saphire, and on the other a Diamond, and so of divers colours. Which may be done after this manner: For example, we would have a Saphire should be white on one side, and blew on the other; or should be white on one side, and red on the other: thus it may be done. Plaister up that side which you would have red or blew, with chalk, and let it be dryed; then commit it to the fire, those ways we spoke of before, and the naked side will lose the colour and turn white, that it will seem a miracle of Nature, to those that know not by how slight an art it may be done.

John Baptist Porta, Natural Magick, 1658

 Compare the previous two accounts to that from nineteenth century Sri Lanka below, which describes the heat treatment of ruby (also quoted by Tennent, 1859):

…the tinge of blue which is frequently found in the stone (giving it the name of neelakantia) is easily removed by burning. The process is simple and is as follows: – The stone is enclosed in a thick coating of chunam [lime] (that which is used by the natives with their betel-leaves) and then exposed to a strong heat. The operation is repeated until the whole of the blue tinge is removed. But care should be taken to subject only such stones as are perfectly free from cracks to this, for one with cracks, if subjected to heat, is said to crumble down in pieces.*

JF Stewart, 1855 (from AM & J Ferguson, 1888)

The above are but a few of the historical mentions of corundum treatments. Far from a recent discovery, ruby and sapphire heat treatment has been practiced for centuries, possibly dating back to Roman times. But today's dramatic face-lifts are a far cry from the subtle changes of the past. It was the discovery that Sri Lanka's geuda sapphires could be revitalized via heat that opened the floodgates. The result has been that virtually all rubies and sapphires traded today have, at some point, been exposed to the heat of an oven. Indeed, certain pieces which would have been worthless a mere two decades ago, can today be transformed into gems worth tens of thousands of dollars.

Modern heat: Enter the geuda

Who was first to discover the potential of the geuda and when did it happen? No one knows for certain when, but it was not until the 19th century that ovens capable of reaching the necessary temperatures (>1500°C) became available. In answering the question of who, the most convincing account is that related to the author by an old-school Paris dealer (who wishes to remain anonymous).

According to this source, modern heat treatment was developed by Professor and Madame Bron of Company Grasset and Bron, rue Chantepoulet, Geneva. Although not confirmed, it is suspected that the Brons were involved in production of Geneva ruby, a forerunner of Verneuil synthetic corundum. Apparently one result of their experiments with the fusion of natural ruby fragments was the secret of high-temperature corundum treatment.

The first stones heated allegedly originated from the Pailin mines in Cambodia. Treated sapphire rough was given for cutting to lapidaries in the Jura Mountains of Europe at least by 1920 (and possibly as early as 1915). Because the rough was being obtained from the Thai-Cambodian border area, the Brons had contact with people there. At one point, a Thai was hired to help with the work and the secret then spread. Prof. Bron eventually died, but Madame Bron continued her husband's work well into the 1950s. Not without a good deal of guilt, too, for, according to my source, she realized that the family business had committed fraud on a grand scale.

Coldham (1992) gives a slightly different version of the tale. According to him, two Cambodian students came to stay at the Swiss gentleman's house, a then-common practice among wealthy French-speaking Cambodians. During their stay, they learned of the process and upon their return to Cambodia, passed the secret on to dealers in the sapphire fields. Coldham also mentions a London dealer who bought quantities of both silky and non-silky Australian sapphire in the late 1960s. Cut stones were then sold back to Australia by the same London dealer. Due to his experience as a cutter, Coldham realized that the volume of clean rough going to London could not account for the volume of cut stones coming back to Australia. He later learned that the London dealer and the Swiss gentleman had close business associations.

An interesting aside to the above is given in an article from 1916, which reported:

It is noteworthy that there has been a strong demand for dark violet-blue stones. These stones are so dark that they appear quite opaque in dull weather, and can only be identified on a cloudless day. In the larger sizes (up to 3 oz. in weight), stones of this colour sell for as much as £5 per oz., although they yield a black stone when cut locally, and it is suspected that the Germans have some method whereby they can modify the colour. It may be suggested that this is probably done by the simple method of heating the stone. Many minerals, such as, for instance, smoky zircon, have their colour modified and their transparency greatly increased after having been heated to redness; and a specimen of Anakie sapphire examined at the Imperial institute showed a greatly increased transparency as a result of this treatment.

Anonymous, 1916
Sapphire-mining industry of Anakie, Queensland

In 1966, Robert Crowningshield described a blue sapphire reportedly from Thailand that displayed a very weak spectrum and a greenish white SW UV fluorescence. Later, he reported on a fine natural blue sapphire of 18 cts that displayed a zoned greenish white fluorescence under SW UV (Crowningshield, 1966, 1970). While Crowningshield mentioned nothing of heat treatment, today we know that such reactions are common in heat-treated sapphires, particularly those from Sri Lanka. It is highly likely that these were early examples of heated sapphires (Beesley, 1982).

From this point on, the facts are known. In the 1970s, someone in Thailand began applying the process to low-grade Sri Lankan material on a large scale. By the latter part of that decade, large numbers of heat-treated Sri Lankan sapphires were streaming out of Thai ovens. In the early days of what amounted to the Great Geuda Rush, rough could be had for a song and fortunes were amassed overnight. The rest, as they say, is history…

R S end dingbat


  • Anonymous (1916) Sapphire-mining industry of Anakie, Queensland. Bulletin of the Imperial Institute, Vol. 14, April-June, pp. 253–261.
  • Ball, S.H. (1950) A Roman Book on Precious Stones. Los Angeles, Gemological Institute of America, 338 pp.
  • Beesley, C.R. (1982) The alchemy of blue sapphire. Jewelers' Circular-Keystone, August, pp. 102–103.
  • Caley, E.R. (1927) The Stockholm Papyrus. Journal of Chemical Education, Vol. 4, No. 8, August, pp. 979–1002.
  • Clément-Mullet, J. (n.d., ca. 1982) Essai sur la Minéralogie Arabe. Reprint of 1868 ed., Amsterdam, APA-Oriental Press, 406 pp.
  • Coldham, T.S. (1992) The Australian sapphire industry. Australian Gemmologist, Vol. 18, No. 4, pp. 104–107.
  • Crowningshield, R. (1966) Developments and Highlights at the Gem Trade Lab in New York: Unusual items encountered [sapphire with unusual fluorescence]. Gems and Gemology, Vol. 12, No. 3, Fall, p. 73.
  • Crowningshield, R. (1970) Developments and Highlights at GIA's Lab in New York: Unusual fluorescence. Gems and Gemology, Vol. 13, No. 4, Winter, pp. 120–122.
  • Ferguson, A.M. and Ferguson, J. (1888) All About Gold, Gems and Pearls in Ceylon and Southern India. Colombo, London, A.M. and J. Ferguson, 2nd edition, 428 pp.
  • Nassau, K. (1984) Gemstone Enhancement. London, Butterworths, 2nd edition, 1994, 252 pp., 221 pp.
  • Porta, J.B. (1658) Natural Magick. [1st English translation, reprinted by Basic Books, New York, 1957], London, Thomas Young and Samuel Speed, 409 pp.
  • Sersen, W.J. (1991) Gemstones and early Arabic writers. Gemological Digest, Vol. 3, No. 2, pp. 34–40.
  • Stewart, J.F. (1855) An account of the gems and gem-men of the district of Saffragam. Colombo Observer, Colombo, June 11.
  • Tennent, E.J. (1859) Ceylon: An Account of the Island, Physical, Historical and Topographical. 1977 reprint, Dehiwala, Sri Lanka, Tisara Prakasakayo Ltd., 2 Vols.


Neelakantia is a Sri Lankan term for a ruby with a trace of blue in its color. The reference to rubies sometimes cracking from heat treatment may refer to the explosion of CO2 inclusions, which are common in Sri Lankan corundums. 

Author's Afterword

This article is based upon an excerpt from my book, Ruby & Sapphire, and appeared in the Australian Gemmologist (1995, Vol. 19, No. 2, pp. 52–54). It derives from a dusty tome I stumbled across containing an account of the heat treatment of Australian sapphires in 1916.


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