Turquoise
CuAl₆(PO₄)₄(OH)₈·4H₂O · Triclinic
Turquoise forms where copper-bearing water percolates through aluminium-rich rock in dry climates. The critical thing to know is commercial rather than geological: the large majority of turquoise sold is stabilised, reconstituted, or dyed howlite, and untreated natural material is uncommon and priced accordingly.
Properties
- Mohs hardness
- 5–6
- Streak
- white, pale green
- Lustre
- waxy, dull
- Colour
- Sky blue to green, usually with a dark brown or black web of host-rock veining known as matrix.
- Cleavage
- No cleavage in the massive material that is always what you find.
- Fracture
- Conchoidal
- Specific gravity
- 2.6–2.9 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm turquoise
- Waxy, slightly porous surface rather than a glassy one.
- Hardness 5–6, so a steel knife barely marks good material and cuts poor material easily.
- A hot needle on an inconspicuous spot will produce a plastic smell on the resin-stabilised or imitation material that dominates the market.
What turquoise gets confused with
Each of these is settled by one test. Run that test first.
Howlite is much softer (3.5) and its dye sits in the veins rather than the body of the stone.
Chrysocolla is softer and usually more translucent.
Variscite is greener and generally lacks turquoise's blue tone.
Where it is found
Arid copper districts — the American Southwest, Iran, and China are the historic sources.
Is it worth anything?
Natural untreated turquoise with good colour is genuinely valuable. Assume any inexpensive turquoise has been treated in some way, because it almost certainly has.