Topaz occurs in a range of different colors deep golden yellow topaz(sometimes called sherry topaz)  and pink topaz are the most valuable; blue and green stones are also popular.  Natural pink stones are rare most pink topaz is heat-treated yellow material.  colorless heat treated to a range of blues,  some almost indistinguishable from aquamarine when seen with the naked eye.  Some topaz has tear-shaped cavities,  containing a gas bubble or several immiscible(non-  mixing)  liquids.  Other inclusions such as cracks,  streaks,  and veils also occur.  Prismatic topaz crystals have a characteristic lozenge-  shaped cross-section and striations parallel to their length.  Topaz has one perfect cleavage.

Crystal Structure Orthorhombic
Composition Aluminum Fluorohydroxysilicate
Colour Yellow, orange, brown, pink to red to purple red, blue , light green and colorless
Luster Vitreous
Refractive Index 1.62 to 1.63
Specific Gravity 3.54
Mohs Hardness 8
DR 0.010