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Amethyst
Amethyst is a purple variety of quartz. In its natural state, amethyst may have a milky appearance or veining and a very intense, opaque color. By heat-treating the stone, the white disappears and the color lightens, making a clearer stone.
Aquamarine
Natural aquamarine may have a yellowish-brown or yellowish-green color. Heat-treating can transform it to more desirable greenish-blue color. Due to the extreme transparency of aquamarine, inclusions or imperfections are highly visible. Heat-treatment reduces the appearance of chemical inclusions but does not remove cracks.
Citrine
Citrine possesses two unique qualities. First, citrine permanently changes colors if exposed to direct sunlight for a day. Second, in rare cases, it grows in conjunction with amethyst, forming a stone known as ametrine. Citrine's pretreatment color is a golden brown. After treatment, citrines range from bright yellow to a deep orange.
Ruby
Rubies form in both transparent and opaque varieties and posses a naturally high level of inclusions. Under magnification, the inclusions have the appearance of silk threads passing through the stone in all possible directions. Heat treatment not only brings out the richness of the ruby's color but also melts the inclusions, producing a clearer stone.
Sapphire
The sapphire naturally occurs in blue, yellow and white varieties. By heat-treating the stones alone or in the presence of chemicals such as beryllium or boron, the stone changes its intensity and possibly the color. White colors become a pale blue with heat alone and an unnaturally intense blue when treated with heat and beryllium. When shopping for sapphires, be aware that a natural sapphire sells for four to five times the price of a heat-treated sapphire.
Tanzanite
Inexperienced gemstone shoppers may mistake a tanzanite for a sapphire due to its rich blue color. However, a tanzanite possesses a slight violet hue and the color noticeably shifts in different lighting. Before heat treatment, natural tanzanite appears muddy with only slight undertones of blue. After heat treatment, the stone loses all brown and becomes a vibrant, almost clear, blue.
Topaz
While commercial topaz is available in yellow, blue or red, natural topaz is a brownish yellow. Blue topaz are produced by irradiation and then heat-treated at 180 degrees Celsius to bring out the blue color. Heat-treatment at 450 degrees Celsius produces a purplish-pink color.
Tourmaline
Tourmaline is an term encompassing 14 different varieties of mineral compositions and colors, including blue, pink, yellow, green, red, colorless and a mixed stone known as watermelon for its combination of green and pink colors. Heat-treating brings out the vibrancy of the colors.
Zircon
Do not confuse zircon with cubic zirconia, which is a synthetic stone. Natural zircon is a dark reddish brown. Heat-treating the stone at different temperatures produces blue, yellow or colorless stones.