Knowledge Base · Gems & Precious Materials

The Quartz Family

One mineral wearing a dozen faces. Quartz is silicon dioxide — SiO₂ — Mohs 7 and one of the most abundant minerals in Earth's crust (second only to feldspar). The whole family is the same chemistry; what changes is crystal size and a trace of impurity. Amethyst is the February birthstone.

Amethyst — macrocrystalline quartz

Split quartz in two and you have the whole family. Where it grows as single crystals large enough to see, you get the transparent gems — amethyst, citrine, smoky, rock crystal — cut with facets to throw light. Where it grows as a dense felt of microscopic crystals, you get chalcedony — agate, carnelian, onyx — cut as smooth cabochons.

Colour is almost always a trace of something else: iron makes amethyst purple and citrine gold; aluminium darkens smoky quartz — most of it switched on by natural irradiation deep underground.

ClassMineral · SiO₂

Hardness7 Mohs

SystemTrigonal

FractureConchoidal

Abundance2nd after feldspar

BirthstoneAmethyst · Feb

Two branches of one mineral

Every quartz gem is the same SiO₂ — the family splits by crystal size: macrocrystalline stones grow as single crystals you can facet; cryptocrystalline chalcedony is a mass of microscopic fibres, cut as cabochons. Colour comes from trace elements (iron, aluminium) switched on by natural irradiation. — GIA / Britannica

Macrocrystalline — the crystal gems

Amethyst

Purple quartz — colour from iron plus natural irradiation; the most prized of the family. February

Citrine

Yellow-to-orange from a trace of iron; natural citrine is rare, so most on the market is heat-treated amethyst. heat-treated

Ametrine

Amethyst and citrine zoned in one crystal — purple and gold together; from Bolivia. Bolivia

Rose Quartz

Soft pink from microscopic fibre inclusions; nearly always massive, so cut as cabochons and beads. massive

Smoky Quartz

Smoky brown to near-black, from aluminium plus natural irradiation; common and inexpensive. worldwide

Rock Crystal

Water-clear, pure SiO₂ — the colourless quartz that gave us the word "crystal". colourless

Chalcedony — the microcrystalline gems

Agate

Chalcedony in conspicuous colour bands; often dyed to brighten — which should be disclosed. banded

Carnelian

Translucent red-to-orange chalcedony, coloured by iron oxide. iron oxide

Chrysoprase

Apple-green chalcedony, coloured by nickel-bearing minerals. nickel

Jasper

Opaque chalcedony in red, brown, green and grey — often boldly patterned. opaque

Onyx

Chalcedony in straight parallel bands, classically black-and-white. banded

Tiger's Eye

Golden-brown quartz with a silky moving band of light (chatoyancy) from parallel fibres. chatoyant

Hard, but disclose the treatments. Mohs 7 — tough enough for everyday wear, with no cleavage (it breaks in curved conchoidal scoops, not flat planes). Treatments are routine and should be stated honestly: most citrine and green "prasiolite" are heat-treated amethyst, and agate is frequently dyed. Strong, prolonged sunlight can slowly pale amethyst, rose quartz and natural citrine, and high heat can change colour — keep treated stones from sun, steam and heat.

Sources

refGIA — Amethyst ↗

refGIA — Citrine ↗

refEncyclopaedia Britannica — "Quartz" ↗

refEncyclopaedia Britannica — "Chalcedony" ↗

Illustrations are stylised, not photographs; colours and patterns are typical of each kind, not to scale. General educational guidance — value is not financial advice.