– the exposure of a gem to high temperatures for the purpose of altering its color and/or clarity.
1. The most commonly encountered heat-treated gems include.
Amber – When amber is submerged in hot oil—linseed oil for example—it's inherent body color can darken, and the material can take on a clearer appearance. The hot oil can also cause the material to develop a series of spangled, glittery inclusions.
The rounded inclusions in amber are caused by immersing it in heated oil, which results in a “spangled” effect.
Amethyst – Heating can remove unwanted brownish inclusions in some amethysts or lighten the color of overly dark stones.
Aquamarine – Without treatment, much of the aquamarine is blue–green in coloration. Heating in a controlled environment can remove the greenish color component from the material to produce a more blue appearance.
Citrine – Some forms of amethyst can be heated and turned into citrine.
Ruby – Heating can remove purplish coloration rendering a more pure red color. The process can also remove “silk” (minute needle–like inclusions) that can cause a gem to appear lighter in tone and be more opaque. Heating can also cause recrystallization of the silk inclusions to make them more prominent which allows the gemstone to have stronger asterism (a reflecting star effect).
Sapphire – Heating can intensify, or even induce, a blue coloration in sapphires. The heating can also remove “silk” inclusions, which also helps to make the material appear more transparent. It can also cause recrystallization of the silk inclusions to make them more prominent, which allows the gemstone to have stronger asterism (a reflecting star effect).
Pale sapphires that were once discarded in the mining process were treated to a desirable color of blue when heated in a controlled environment.
Tanzanite – The mineral zoisite, which includes the gem variety known as tanzanite, it is often heated at low temperatures to remove a brownish color component to produce a stronger purplish-blue color.
Tanzanite often is mined as a brownish material (as the rough and cut stone on the left show). Once heated, the gem changes to a blue or purplish blue color (as the rough and cut gem on the right show).
Topaz – Heating yellowish pink topaz sometimes has the effect of removing the yellowish color component, thereby intensifying the pink color. Heating is also used to control the color of blue topaz—the material that may have started out as colorless is irradiated followed by heating which results in a desirable blue color.
This imperial topaz crystal was sawn in half. The crystal on the right was heated resulting in the purplish pink color. Both colors are highly desired in the market.
Tourmaline – Sometimes heat treating can cause overly dark green material to become lighter in tone, or it may affect the color in other tourmalines.
Zircon – Some reddish brown zircons are heated in controlled environments to produce more commercially viable colors, including an intense blue.
2. Durability factors – Heat treatments in all of the gemstones mentioned above are considered durable and permanent under normal handling conditions.
3. Special care requirements – Submitting gemstones to intense heat may render them slightly more brittle than usual, and care must be taken not to damage pointed faceted corners and edges.
Source: Robert Weldon, www.gia.edu
To learn more about gem treatments see also: