Gemology Fast Track / Unit 16 of 24 · Major species · 60 min
Feldspar, peridot, tanzanite, iolite, spodumene
Goal: Identify the remaining big commercial gems and their care issues.
Peridot
1.654–1.690RI
0.036B+
3.34SG
Strong doubling, lily-pad inclusions, Fe spectrum 453/477/495. Idiochromatic (only green).
Tanzanite
1.691–1.700RI
0.009B+
3.35SG
Heated zoisite. Trichroic. One perfect cleavage. Only Merelani, Tanzania.
Iolite
1.542–1.551RI
0.008–0.012B−
2.61SG
Violet-blue / yellow / grey trichroism visible by eye.
Spodumene
1.660–1.676RI
0.015B+
3.18SG
Kunzite (pink, fades), hiddenite (green). Two perfect cleavages.
Moonstone
1.518–1.526RI
B−orthoclase
2.57SG
Adularescence; centipede inclusions. Rainbow moonstone is labradorite (RI 1.56).
Labradorite / sunstone
1.54–1.57RI
B±plagioclase
2.62–2.74SG
Labradorescence; aventurescence (Cu or hematite platelets).
Pleochroism you can see
Iolite
Tanzanite (unheated)
Andalusite
Kunzite
Separation traps
Tanzanite vs synthetic forsteriteForsterite bire 0.033–0.035 → strong doubling; tanzanite 0.009.
Tanzanite vs ioliteRI 1.69 vs 1.54.
Peridot vs green tourmalinePeridot B+ bire 0.036 SG 3.34; tourmaline U− 0.020 SG 3.06.
Andesine 'red feldspar'Most is Cu-diffused; assume treated without documentation.
Care: tanzanite, kunzite, moonstone, peridot — no ultrasonic, no sudden heat. Kunzite fades in sunlight.
Gems in this unit (22)
Practical
If you have any blue/violet stone, check for pleochroism with your dichroscope or by rotating under a single bright light, and match to the strips above.
Unit quiz
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