Final answer:
Heat treatment is done to corundum that has not been disclosed as treated, and the practice of revealing gemstone treatments is essential to prevent asymmetric information and maintain market trust.
Step-by-step explanation:
Heat treatment is typically done to corundum that has not yet been disclosed as treated. When gemstones such as emeralds are sold, it is important that any treatments like oiling or adding epoxy are made known to the buyer. This ensures transparency and prevents a situation of asymmetric information, where the seller may know more about the gemstone's treatments than the buyer. The case of emeralds in New York City in 1997 highlights the issue where sales clerks claimed the stones were untreated, but lab testing showed otherwise. Similar treatment methods are applied to other gemstones like diamonds, topaz, and tourmaline, often through irradiation to enhance colors. It is essential for these gemstone treatments to be openly communicated to maintain trust in the gemstone market.