Quartz.

Quartz is silicon dioxide, written SiO2, and at roughly twelve percent of the earth's crust by mass it is the second most abundant mineral after feldspar. It crystallises in the trigonal system as a framework silicate (a tectosilicate), with each silicon atom bonded to four oxygens, each oxygen shared between two silicons, the whole thing wound into a helical structure that is either right-handed or left-handed. That handedness is what makes quartz piezoelectric: squeeze a slice of it the right way and a small voltage appears across the faces, the property that runs every quartz watch and every old radio oscillator on the planet.

The family of colour varieties

Most of the named "quartzes" sold as crystals are colour varieties of the same mineral. Amethyst is violet quartz, coloured by trace iron (Fe4+) in the lattice activated by natural gamma irradiation underground; heated to around 470 to 750 degrees Celsius it turns yellow to orange and is then sold as citrine. Natural citrine is yellow to honey from iron in a different oxidation state and is genuinely uncommon, which is why most commercial citrine is heat-treated amethyst from Brazil. Smoky quartz is grey to brown to nearly black, coloured by aluminium impurities and natural irradiation; morion is the very dark variety. Rose quartz is pink, usually from microscopic fibres of dumortierite or a related borosilicate scattering light, with a smaller contribution from trace titanium. Pure clear quartz is the colourless original, sometimes called rock crystal in older trade language. Other named varieties include prasiolite (pale green, almost always heat-treated amethyst), aventurine (quartz with mica or fuchsite inclusions), and milky quartz (clouded by tiny fluid inclusions). Chalcedony, agate, jasper, onyx, carnelian, and chrysoprase are all cryptocrystalline quartz: the same chemistry, but built of grains too small to see, and so without the prismatic habit.

The mineral sits at 7 on the Mohs scale, which makes it durable enough for everyday wear and the threshold above which a stone resists abrasion from common dust. It has no cleavage and breaks with a conchoidal fracture, which is why quartz pebbles tumble so smoothly in rivers. It typically grows as six-sided prisms ending in pyramidal tips, with horizontal striations across the prism faces and a left- or right-handed twist visible in well-formed crystals. Massive (un-pointed) forms are common, especially for rose quartz, which rarely grows as good points outside Brazil's Minas Gerais.

In hand and in practice

Inclusions are part of the appeal. Rutile threads, chlorite ghosts, hematite phantoms, and black tourmaline needles all show up inside quartz and turn each piece into a small geological record. Phantoms (an older crystal layered inside a younger one) are common in amethyst, marking pauses and resumptions of growth. Two-phase inclusions, where a tiny fluid bubble can be seen moving when the stone is gently warmed, are diagnostic for natural origin.

A common misconception is that all yellow quartz is citrine, regardless of how it got that way. Strictly, gemmologists distinguish natural citrine from heat-treated material, and the prices on a genuine natural Madagascan or Russian citrine are an order of magnitude higher than for the Brazilian heat-treated stock that fills most shops. Another misconception is that synthetic quartz, grown hydrothermally for the watch and electronics industries, is somehow inferior to natural quartz. Chemically and structurally it is identical. What differs is provenance and inclusions, which is what most crystal buyers are paying for.

In modern crystal practice the quartz family is the workhorse. Clear quartz is treated as a general amplifier, and the coloured varieties take on the meanings their hues suggest: rose for the heart, smoky for grounding, amethyst for calm thinking, citrine for confidence and warmth. The breadth of the family is part of why quartz dominates the entry-level shelf and the experienced collector's drawer alike.