An acceptable situation - HEAT TREATMENT

What do you tell the customer who asks, "Has this gemstone been treated?"
Depending on your answer, that customer will either walk out the door with a bad taste for the jewelry industry or with a greater understanding and appreciation for the beauty of gemstones - and possibly the stone you were showing them.
The improvement of a gemstone's color through a heating process is an everyday occurrence in the gem centers of the world. In Thailand, Sri Lanka, Australia, Brazil - everywhere a gem occurs, there is a treatment to enhance and stabilize the color and bring out the hidden potentials of a stone. Without it, there would be few examples of beautiful gemstones we are all aware of. Conversely, processes such as irradiation and diffusion create color that a stone had no potential to attain and are therefore treatments looked down upon in the trade.
There are several reasons to improve a gemstones color through heating. Light stones may be darkened, dark stones may be lightened. Asterism may be enhance or eliminated.
To accurately answer the opening question, a jeweler can only reply, "probably." Since the process of heating a gem species occurs naturally, man's introduction of heat is merely an extension of nature. In most cases, such treatment is not detectable by a gemologist and therefore cannot be disclosed on a laboratory report. Heating has most assuredly occurred, but the characteristics which identify heating are identical by both man and nature.
The process of heat treatment can only bring out characteristics that a stone already possesses. Pink sapphire will not become gem ruby. The color change in alexandrite will not be appreciably enhanced. Heat treatment has thereby gained acceptance in the jewelry industry. Heat diffusion and other techniques which create a coating to induce color, however, are not acceptable to the trade unless acknowledged and disclosed to the customer.
Heat treatment techniques vary with each gem species as temperature and time expose the important factors. In some cases, solutions are added to the process to facilitate coloration.
Examples of commonly treated gemstones include:

• Aquamarine, out of the ground as green to yellowish-brown beryl, when heated to 400-450 degrees Celsius to produce that familiar light to medium blue "aqua" coloration.
• Reddish brown topaz becomes a salmon-pink or purplish-red.
• Zoisite, which occurs as a muddy greenish-brown is transformed to beautiful highly pleochroic violetish-blue tanzanite.
• Dark blue Australian sapphires are lightened, although a more difficult process, through prolonged heating. Light blue to near-colorless Sri Lanka sapphires gain color through extreme heating.
• Dark green tourmaline is reduced in tone to a pleasing medium-green.

Heat treatment is not without its pitfalls. Sapphire is most often heated to just below its melting point, making the process all the more difficult. Much money has been lost through "overbaking" to a point of ruining a stone.
If you're still stuck for an answer to a question "Has this gemstone been treated?" liken the process cookies - the dough's OK, but wait til they come out of the oven!