Rock IdentifierThe Quartz Family
Key takeaways
- Quartz is silicon dioxide (SiO₂), Mohs 7, with no cleavage and conchoidal fracture.
- It splits into macrocrystalline (amethyst, citrine) and microcrystalline (agate, jasper) branches.
- Its hardness and glassy luster make it easy to separate from softer look-alikes like calcite.
The everywhere mineral
Quartz is one of the most abundant minerals in Earth's crust and by far the most common gem material. Chemically it is simply silicon dioxide (SiO₂), but trace impurities and growth conditions produce an enormous family of colors and textures — from water-clear points to banded agate. Learning quartz well is a shortcut to identifying a huge share of the specimens people find.
How to recognize it
Three properties nail it down: a hardness of 7 (it scratches glass and steel easily), a glassy (vitreous) luster, and no cleavage — broken pieces show smooth, curved conchoidal fractures rather than flat faces. Well-formed crystals grow as six-sided prisms capped by a point. Together these separate quartz from softer imposters.
Two big branches
Macrocrystalline quartz has visible crystals: clear rock crystal, purple amethyst, yellow citrine, pink rose quartz, grey-brown smoky quartz. Microcrystalline (cryptocrystalline) quartz is made of crystals too small to see and forms chalcedony — including agate, carnelian, onyx, and the opaque, grainy jasper.
Look-alikes & value
Calcite is the classic confusion, but it's much softer (3), has rhombic cleavage, and fizzes in acid — quartz does none of that. Glass can mimic clear quartz but often traps round bubbles and lacks natural crystal faces. Common quartz is inexpensive; value rises with fine color, clarity, unusual habit, or a notable locality.
