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Moh’s hardness: 7
Refractive index: 1.544-1.553
Critical angle: 40°
Specific gravity: 2.63-2.65
Fracture: Conchoidal, brittle.
Dispersion: Medium (.013)
Heat sensitivity: Low
Pleochroism: Amethyst: very weak (purple to grayish purple); Natural citrine: weak (yellow to light-yellow); Prasiolite: very weak (light-green to paler green); Rock crystal: colorless, no dichroism; Rose quartz: weak (pink to paler pink); Natural dark smoky: definite (brown to reddish-brown).
Birefringence (double refraction): Weak (.009)
Crystal structure: Hexagonal
Treatments: Oro Verde, lemon citrine, smoketrine and most dark gray smoky quartz without any brown tones are all irradiated. Citrines are often heat treated to improve color and give a reddish tint. Most citrine is amethyst that has been heat treated. Some smoky can be heated to create citrine. Prasiolite is heat treated amethyst and citrine from one locality.
Cutting: Quartz cuts easily. Pre-polishing with 1200 Nubond or 3000 standard lap will make polishing much quicker and more enjoyable.
Polishing: Cerium oxide on most common polishing laps.
Comments: Blue colored quartz is available as a synthetic. Beware that nearly a quarter of all commercial cut amethyst is synthetic. Synthetic citrine and ametrine are also being sold as natural by unscrupulous dealers.