Quartz

What holds the family together

Every stone on this page is silicon dioxide, SiO2, crystallised in the trigonal system at hardness 7 with no cleavage. That last point matters more than it sounds: a mineral without cleavage does not split along a plane when you drop it, which is why quartz varieties survive being carried for thousands of years and why so much of the archaeological record is made of them.

What differs is what got in. Trace iron plus natural radiation gives amethyst its violet. Heat that same iron and you get the yellow-orange of citrine. Different iron chemistry gives smoky quartz, which is actually radiation damage to aluminium impurities. Microscopic borosilicate fibres scatter light to give rose quartz its cloudy pink. Titanium oxide needles growing through the crystal give rutilated quartz. A fibrous blue mineral replaced molecule by molecule while keeping its structure gives tiger's eye.

A practice that assigns eight sharply different meanings to eight faces of one mineral is describing human perception rather than matter. That is worth knowing while you use it, and it does not make the practice worthless.

How it forms

Quartz crystallises late from silica-rich fluids, which is why it lines cavities, fills veins and forms the last mineral in a cooling granite. Good crystals need space and time: a pocket in a pegmatite, a vug in a lava flow, a fissure in the Alps opened by tectonic movement.

The colour varieties tend to have specific geological addresses. Amethyst forms in gas bubbles in basalt, which is why the great Brazilian and Uruguayan geodes are volcanic. Smoky quartz forms near granite with enough radioactive minerals to supply the dose. Rose quartz almost always occurs as massive material in pegmatites rather than as crystals, which is why terminated rose quartz points are rare and expensive.

The famous specimens follow the geology. Herkimer diamonds are doubly terminated quartz crystals that grew free in cavities in dolostone in New York, so they have no attachment point at all.

The stones of this family

• Clear quartz (SiO₂, Mohs 7). The stone every tradition kept and nobody agrees about. Piezoelectric, abundant, and used as a scrying surface long before it was called a master healer.
• Amethyst (SiO₂ with iron, Mohs 7). Purple quartz, named from the Greek for not drunken. Bishops' rings, Russian mines and February's birthstone, and the most misunderstood colour in the shop.
• Rose quartz (SiO₂ with titanium, Mohs 7). Pink from microscopic mineral fibres, which is why it is almost never transparent. Beads from Mesopotamia around 7000 BCE, and now the stone of self-kindness.
• Citrine (SiO₂ with iron, Mohs 7). Genuinely rare, and almost everything sold under the name is heated amethyst. Learning to tell them apart is the fastest lesson in buying honestly.
• Smoky quartz (SiO₂, irradiated, Mohs 7). Brown quartz coloured by natural radiation in the ground. The national stone of Scotland, worn in the hilt of the dirk and the shoulder brooch.
• Rutilated quartz (SiO₂ with rutile needles, Mohs 7). Clear quartz shot through with golden needles of titanium oxide, once called Venus hair. The inclusion is the point, not the flaw.
• Green aventurine (quartzite with fuchsite, Mohs 7). A rock rather than a single crystal, glittering from mica flakes. The gambler's stone in modern lore, which is a claim worth handling carefully.
• Tiger's eye (SiO₂ after crocidolite, Mohs 7). Quartz that replaced a fibrous blue mineral and kept its silky bands. Roman soldiers carried it engraved; the chatoyant flash gave it the eye of every culture's watchful animal.

Trade names and lookalikes

The single most useful fact in this family is that most citrine is heated amethyst. It is not fraud when disclosed, and it is very often not disclosed. Heated material runs orange-brown with the colour concentrated at the tips and often a bleached white base; natural citrine is a paler, more even lemon.

Aura quartz, in every metallic rainbow variant, is real quartz with a vapour-deposited metal film applied in a vacuum chamber. It is a technology from the 1980s. Any page describing an ancient meaning for it has invented one.

Prasiolite, the green quartz, is nearly always heat-treated amethyst. Rose quartz is imitated with pink glass. Herkimer diamond is a trade name for a locality-specific quartz, not a diamond of any kind.

Synthetic quartz has been grown industrially since the 1950s, in enormous quantity, because the electronics industry needs it. Some finds its way into jewellery, honestly and otherwise.

Care that follows from the structure

Hardness 7 and no cleavage means quartz is the most forgiving family in the catalogue. It survives pockets, keyrings and daily rings, it is safe in water, and it can be cleaned with warm water and a soft brush.

Two cautions apply. The first is light: amethyst, rose quartz, citrine and smoky quartz all fade in sunlight, permanently. Whoever popularised charging crystals on a sunny windowsill was not thinking about colour centres. Moonlight is harmless; the problem is the morning.

The second is heat. A few hundred degrees will turn amethyst yellow. That means no hot ultrasonic cleaners for coloured quartz, and no leaving a piece on a radiator or a car dashboard.

Rutilated quartz has one extra consideration: the rutile needles reach the surface in places and can be plucked out by an aggressive polish or a rough setting.