Gemology Fast Track / Unit 12 of 24 · Major species · 60 min

Spinel and chrysoberyl

Goal: Identify spinel (and separate synthetics) and the chrysoberyl family including alexandrite.

Spinel

8H
1.712–1.720RI · SR
3.60SG

Octahedral crystals; octahedral negative crystals often in rows. Red/pink: Cr, strong red fluorescence, 'organ-pipe' emission lines. Not routinely treated → strong demand.

Chrysoberyl

8.5H
1.746–1.755RI · B+
3.73SG

Yellow-green chrysoberyl; cat's-eye (cymophane); alexandrite (Cr, colour change). Very bright, tough.

Natural vs synthetic spinel

NaturalRI 1.712–1.720 (gahnospinel higher), SG 3.58–3.61, little strain, octahedral inclusions.
Flame-fusion syntheticRI 1.727–1.730, SG 3.64, strong 'tabby' ADR, gas bubbles. Mostly imitates aquamarine, blue sapphire, peridot — not spinel.
Flux syntheticOverlaps natural; flux, Pt platelets; lab photoluminescence helps.
Cobalt blueNatural Co-spinel exists (Vietnam, Sri Lanka) but most vivid Co-blues on the market are synthetic. CF red; Co bands.

Alexandrite colour change

Daylight / LED 6500 K
→
Incandescent 2700 K

Grade the change

% change (0–100) and quality of each colour. Best: emerald-like green to raspberry red, change ≥ 80%.

Natural signs

Fingerprints, twin planes, fluid inclusions, mica (Brazil), trichroism.

Fakes

V-doped synthetic corundum (RI 1.76–1.77, purple→pinkish, tourist trap), synthetic alexandrite (flux/Czochralski), colour-change garnet (SR).

Cat's-eye chrysoberyl

Silky tubes in one direction; 'milk and honey' effect: lit from the side, one half milky, the other honey. A sharp, straight, centred eye that opens and closes as you rotate is the grade.

Gems in this unit (16)
ChrysoberylAlexandriteCat's-eye chrysoberylAlexandrite cat's eyeRed spinelPink spinelBlue spinelPurple spinelGrey spinelBlack spinelGahnospinelGahniteSynthetic alexandriteSynthetic spinel (flame fusion)Synthetic spinel (flux)Soudé emerald

Full gem records open in the members' Gem database.

Practical
Find a 'spinel twin' and 'alexandrite colour change' image. If you have a red stone, run it in the ID Helper as SR 1.718 SG 3.60 vs DR 1.762/1.770 SG 4.00 and note the difference.
Unit quiz

0/7 answered

1. Flame-fusion synthetic spinel RI and SG:
2. Alexandrite belongs to…
3. A 'colour change alexandrite' with RI 1.762–1.770 is…
4. Chrysoberyl optic character:
5. 'Organ-pipe' fluorescence lines indicate…
6. The only species that can be called simply 'cat's eye'…
7. Natural spinel is usually…
Unit 11Unit 13