Topaz

Topaz is an aluminum silicate mineral containing fluorine and hydroxyl, Al₂SiO₄(F,OH)₂, that crystallizes in the late stages of granitic magma cooling. Topaz forms in the final stages of granitic magma cooling, when fluorine-rich gases and fluids penetrate the surrounding rock. It crystallizes in pegmatites, high-temperature quartz veins, and vugs (cavities) within granite and rhyolite. The fluorine content in topaz's chemistry is what distinguishes it from other aluminum silicates.

The most valued variety - imperial topaz - comes almost exclusively from Ouro Preto in Brazil's Minas Gerais state. These orange to pinkish-orange stones get their color from chromium, and the specific geological conditions at Ouro Preto have never been replicated elsewhere. Natural pink topaz is also extremely rare and is found primarily in Pakistan and Russia.

Almost all blue topaz on the market is irradiated and heated colorless topaz. Natural blue topaz exists but is extremely pale. The vivid 'Swiss Blue' and deep 'London Blue' commercial varieties are 100% treated. This is universally disclosed and accepted in the gem trade.

Topaz has perfect basal cleavage in one direction - it splits cleanly into flat sheets. This is its most diagnostic physical property and also its biggest vulnerability (topaz can crack if dropped on a hard surface). At hardness 8, it's harder than quartz.

Distinguish from citrine (lower hardness at 7, no cleavage), aquamarine (lower SG, different crystal habit), and blue glass (no cleavage, may have bubbles). The combination of hardness 8 plus perfect basal cleavage is essentially unique to topaz among common gems.