oxides
Quartz
SiO2
- Also known as
- rock crystalsilica
- Crystal system
- trigonal
- Hardness
- 7 on the Mohs scale
- Lustre
- vitreous
- Colour
- Colourless when pure; almost any colour with impurities
- Where it forms
- Almost everywhere. It crystallises from silica-rich melts, precipitates from hot water in veins, and survives weathering when the minerals around it do not — which is why most sand is quartz.
- Chiefly from
- BrazilMadagascarArkansas, United Statesthe Alps
Why it is that colour
Pure quartz is water-clear. Every coloured variety in this collection is the same compound carrying a trace of something else, or carrying a structural defect produced by natural radiation, usually at concentrations far too low to change the formula.
Safety
Harmless to handle. Cutting or grinding it is a different matter: respirable crystalline silica dust causes silicosis and is a recognised occupational carcinogen, which is a lapidary and stonemasonry hazard rather than a shelf one.
Where it comes from
Enormously abundant and mined worldwide. Brazil is the largest source of specimen and lapidary material; concerns about informal and unregulated small-scale mining recur there and in Madagascar.
Historical use
Worked for tools before it was worn as ornament — quartz and its cryptocrystalline forms are among the oldest worked materials on earth. Later used for lenses, seals and vessels. Its piezoelectric property, discovered by the Curie brothers in 1880, put it in radios, watches and sonar, and this genuine industrial fact is routinely borrowed as evidence for effects it has nothing to do with.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
Rock crystal was understood in the Greek world as water frozen so hard it could not thaw — the word krystallos meant ice first and quartz second, and Pliny records the belief as established fact in the first century. Its modern role as an amplifier of other stones is much younger and belongs to the 1970s and 80s literature.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
The piezoelectric argument deserves stating precisely, because it is the one piece of real physics in the field and it is almost always misused. Quartz does generate a small electrical charge when it is mechanically deformed, and that is why a quartz watch keeps time: a tiny bar of it is made to vibrate at a precise frequency by an applied voltage. What the effect requires is mechanical stress and a circuit. A crystal resting on a table, or on a person, is under neither, and would produce nothing biologically meaningful if it were.
The strange thing is that the real story is more impressive. A sliver of this rock is the reason your watch is accurate, and nobody had to invent anything about it.
Identity checked against IMA-CNMNC master list, June 2025.