Heat treatment exposes gems to controlled temperatures, often 500–1800 °C, in oxidizing or reducing atmospheres to alter color or improve clarity by dissolving or modifying inclusions. It is permanent in most cases and widely accepted for ruby, sapphire, tanzanite, zircon, and aquamarine.
For corundum, heating can dissolve rutile silk (improving clarity) or adjust iron-titanium charge transfer for blue sapphire. Most commercial rubies and sapphires are heat-treated. Unheated stones of comparable quality command substantial premiums (sometimes 2–10 times higher) because of greater rarity. Gemologists identify heat treatment through altered inclusion appearances (e.g., rounded gas bubbles or "fingerprint" patterns) and changes in UV fluorescence.