Quartz.

Quartz is silicon dioxide, written SiO2, and it is one of the most common minerals in the earth's crust. Most of the named "quartzes" sold as crystals are colour varieties of the same mineral. Amethyst is violet quartz coloured by trace iron and natural irradiation. Citrine is yellow to honey, smoky quartz is grey to brown, rose quartz is pink (usually from trace titanium or microscopic fibrous inclusions), and clear quartz is the colourless original.

The mineral sits at 7 on the Mohs scale, which makes it durable enough for everyday wear. It typically grows as six-sided prisms ending in pyramidal tips, though massive (un-pointed) forms are common, especially for rose quartz. Inclusions are part of the appeal: rutile threads, chlorite ghosts, hematite phantoms, and tourmaline needles all show up inside quartz and turn each piece into a small geological record.

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.