Can tanzanite be heat-treated?

Yes, almost all tanzanite you see on the market is heat-treated to coax that well-known, vibrant violet blue appearance. In the rough, unheated state, tanzanite usually shows muted brownish, greenish, or burgundy-looking tones.

The method is basically gently baking the stones at about 600°C (1,100°F). This low-temperature warming is permanent, and it underpins the whole tanzanite trade, since genuine, naturally heated blue crystals sitting near the Earth’s surface are extremely uncommon. Because this treatment is so common, largely unchangeable, and in practice hard to detect, it generally does not reduce the gemstone’s value. Instead, the assessment stays on the final colour saturation, the clarity grade, and the cut quality.