Heat treatment is the most frequent treatment and without heat treatment, the availability of fine gems would be rather less. With less gems, prices of the unheated gems would be only reachable to the rich. This is the reason untreated gems often rule 30% to 50% higher prices. Sapphires, rubies, aquamarine, tanzanite, amethyst, citrine, blue topaz, blue zircon, tourmaline, and others are consistently treated with heat. The time and temperature required to change color varies dramatically from 450 degrees fahrenheit to 1250 degrees are commonly used to alter color. Heat treatment is nearly all commonly used to alter color but is also used to increase transparency. Rubies and sapphire can require considerably higher temperature to alter color and change clarity. Heating is used to either lighten, darken or alter the color of a gem. Amethyst is heated to brighten up its color. After you heat a little more and with some sources of amethyst will alter color from deep purple to orange colored citrine. Uraguay is a fine example. Heat a little longer and amethyst can go colorless or milky, Neither of these are desirable. Tanzanite is heated to around 1000 degrees fahrenheit. Tanzanite is a trichroic gem with blue, violet and yellow colors which in the rough appears as a color familiar in the trade as "diesel". If you look into a can of diesel fuel this is what fine tanzanite rough looks like, a brownish bluish red color. Upon heating the gem is left with the blue and violet we identify as tanzanite. The yellow is now and forever gone. Aquamarine heating is also removing the yellow component which gives natural aqua a green tint. After heating the gem is left without the green giving it a blue color of similar intensity. Green tourmaline is repeatedly heated to lighten color in over saturated gems, like those regularly seen in Brazil. Other tourmalines are heated to alter one of the dichroic colors. Sapphire is one of the few gems which the color can be darkened by heating as seen in Ceylon Sapphire. Grey silky material known as Geuda can be changed to blue by heating. Not only do the rutile needles that cause the cloudiness vanish, these needles are also the blue coloring agent , titanium, that now upon heating produces a blue clear gem sapphire. Ruby heating alter color and enhances clarity. Many burmese rubies from Mong Shu have a blue core and heating increases the blue leaving excellent red gems. The heating may also alter cracks in the ruby by formation of ruby in the cracks due to very extreme heating close to the melt temperature of aluminum oxide. Emerald, garnet, peridot, chrome tourmaline, opals, alexandrite, ametrine, heliodore, kunzite, zoltanite and most spinels are not commonly heat treated. Heat treatment is a very chancy process as many gems have inclusions that enlarge at a different rate and cause stress fractures, which then becomes an identifying characteristic of heated gems, particularly sapphire and ruby which can display halos around inclusions inside the gem. The price adjustment for natural gems is based on someone's ability to establish the treatment which sometimes is quite difficult and beyond most jewelers experience. Ruby is highly valued for being natural and can command more that 50% greater value than a similar treated gem. Unheated Sapphires command an estimated 30% premium over identical heated gems. Natural ruby and sapphires are more durable as they tend to have greater resistance to chipping. Other gems usually bring 10-20% higher value when of natural color.
Author Resource:-
Preferred Gemstones has been cutting gemstones since 2006 and has been a fan of unique design in brilliantly faceted Loose Gemstones. This article is the opinion of the writer only.
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