Quartz
SiO₂ · Trigonal · also called rock crystal, clear quartz
Quartz is the most abundant mineral in the Earth's crust and the one every rockhound learns first. Its combination of hardness 7, complete absence of cleavage, and glassy conchoidal fracture is shared by almost nothing else common. Once you can recognise quartz reliably, a surprising fraction of unknown rocks resolve themselves.
Properties
- Mohs hardness
- 7
- Streak
- white, colorless
- Lustre
- vitreous, greasy
- Colour
- Quartz comes in almost every colour, which is why colour is useless for identifying it. Trust hardness, the lack of cleavage, and the six-sided crystal form instead.
- Cleavage
- No cleavage at all. This is the most useful single fact about quartz — it never splits along flat planes.
- Fracture
- Conchoidal — smooth, curved, shell-like breaks with sharp edges
- Specific gravity
- 2.65 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm quartz
- Scratches glass and steel cleanly. A steel nail slides off without leaving a mark.
- Curved conchoidal fracture surfaces, never flat cleavage faces.
- Well-formed crystals are six-sided prisms capped by a six-faced point.
What quartz gets confused with
Each of these is settled by one test. Run that test first.
Calcite is soft (3), fizzes in vinegar, and splits into rhombs. Quartz does none of these.
Feldspar shows two flat cleavage faces meeting near 90°. Quartz has no flat faces from breaking.
Manufactured glass often contains bubbles and is softer (about 5.5) — quartz scratches it.
Varieties
- Amethyst
- Purple, coloured by iron and natural irradiation. Fades in strong sunlight.
- Citrine
- Yellow to orange. Most commercial citrine is heat-treated amethyst.
- Rose quartz
- Pink, almost always massive rather than in crystals.
- Smoky quartz
- Brown to near black, common in granite pegmatites.
- Milky quartz
- White and cloudy from countless fluid inclusions. The commonest vein quartz.
Where it is found
Everywhere. Granite, sandstone, veins, beach sand, and stream gravel. Vein quartz is the white blocky material littering fields in much of the world.
Is it worth anything?
Ordinary quartz is worthless in cash terms. Well-terminated clear crystals, amethyst of good colour, and quartz with attractive inclusions all have collector markets.