Gemology Fast Track / Unit 15 of 24 · Major species · 60 min

Quartz, chalcedony, opal

Goal: Cover the silica family: crystal quartz, cryptocrystalline chalcedony, amorphous opal.

Quartz (crystalline)

1.544–1.553RI
0.009U+
2.65SG

Bull's-eye figure. Amethyst, citrine, ametrine, rose, smoky, rock crystal, prasiolite.

Chalcedony

~1.53–1.54spot
AGGlight in polariscope
2.60SG

Porous: takes dye. Agate, onyx, carnelian, chrysoprase, jasper.

Opal

1.37–1.47RI
SRoften ADR
1.98–2.20SG

Contains water. Play-of-colour from ordered silica spheres.

Amethyst & citrine tells

Natural amethystAngular colour zoning, Brazil-law twin ('tiger stripes'), Airy spiral figure.
Synthetic amethystBread-crumbs, seed plate, often no twinning; labs use FTIR.
Heated 'citrine'Reddish-orange, no dichroism, amethyst-type zoning.
Natural citrinePale yellow with smoky tint, weak dichroism.

Opal grading

Body tone

N1 (black) → N9 (white). Darker body = more vivid colour = higher value.

Brightness

Scale 1–5: dull → brilliant. Biggest value factor after tone.

Pattern

Harlequin (rare) > broad flash, rolling flash > pinfire.

Colour

Red-dominant > orange > green > blue. Check all angles.

Opal fakes and risks

Doublet / tripletGlue line on side, flat black back, glassy dome on triplet.
Synthetic (Gilson)Lizard-skin / columnar pattern inside colour patches.
Ethiopian hydrophaneAbsorbs water/oil: turns clear, then stains. Smoke-treated to black.
CrazingNetwork of cracks from dehydration; ask how long it's been out of the ground.

Chalcedony dyes

Black onyx, blue/green/pink agates, most 'carnelian' are dyed. Dyed green chalcedony shows Cr lines and turns red under CF; natural chrysoprase (Ni) stays green.

Gems in this unit (49)
AmethystCitrineAmetrineLemon quartzRose quartzSmoky quartzRock crystalHerkimer quartzPrasioliteRutilated quartzTourmalinated quartzCat's-eye quartzLodoliteStrawberry quartzTiger's eyeHawk's eyeAventurine quartzPraseChalcedonyCarnelianSardChrysopraseChrome chalcedonyAgateMoss agateFire agateOnyxSardonyxBloodstoneJasperGem silicaPetrified woodWhite opalBlack opalBoulder opalCrystal opalFire opalEthiopian opalMatrix opalCommon opalPeruvian opalHyaliteCopalUnakiteSynthetic quartzSynthetic opalImitation opalOpal doublet / tripletSoudé emerald

Full gem records open in the members' Gem database.

Practical
Buy a cheap amethyst and examine for colour zoning. Check any opal you own from the side for a join line.
Unit quiz

0/8 answered

1. Quartz optic character:
2. Most commercial citrine is…
3. Chalcedony in the polariscope:
4. Lizard-skin patterns in opal colour patches indicate…
5. Opal body tone N1 means…
6. Natural chrysoprase under a Chelsea filter:
7. Ethiopian Welo opal is often…
8. Brazil-law twinning is typical of…
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