Topaz
Al₂SiO₄(F,OH)₂ · Orthorhombic
Topaz is a pegmatite mineral, hard enough to scratch quartz and heavy enough to feel wrong in the hand. Its single perfect cleavage is both its identifying feature and its weakness — a sharp knock in the right direction will split a fine crystal cleanly in two.
Properties
- Mohs hardness
- 8
- Streak
- colorless, white
- Lustre
- vitreous
- Colour
- Naturally colourless, pale blue, or sherry brown. Almost all vivid blue topaz on the market is irradiated and heat-treated colourless material.
- Cleavage
- Perfect basal cleavage — a single flat plane across the crystal. This is what separates it from quartz at a glance.
- Fracture
- Conchoidal
- Specific gravity
- 3.49–3.57 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Some specimens fluoresce
Tests that confirm topaz
- Hardness 8 — it scratches quartz, which no common mineral does.
- One perfect cleavage plane, often visible as an internal reflective flaw.
- Noticeably heavier than a quartz crystal of the same size.
What topaz gets confused with
Each of these is settled by one test. Run that test first.
Where it is found
Granite pegmatites, rhyolite cavities, and the stream gravels below them.
Is it worth anything?
Imperial topaz (natural orange-pink) is valuable. Blue topaz is inexpensive because it is produced by treatment in large quantities.