By Karen G. Blaettler – Updated August 30, 2022
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Quartz and diamond crystals reveal their identity through distinct crystal shapes, density, refractive index, hardness, and cleavage. Quartz forms six‑sided, elongated crystals with a single terminating end, while diamonds grow as eight‑sided, isometric crystals that are roughly equal in length and width. These differences—combined with their unique physical properties—enable reliable identification, whether using non‑destructive tests or, when necessary, destructive methods.
In nature, the internal arrangement of atoms dictates crystal growth. Quartz, composed of silicon dioxide (SiO₂), crystallizes in a hexagonal lattice, producing long, slender prisms that typically terminate in a single hexagonal pyramid. The rare Herkimer “diamond” quartz can feature termination at both ends. By contrast, diamond’s pure carbon lattice forms cubic (isometric) crystals. These eight‑sided shapes often appear as two pyramids joined at their bases, with all dimensions nearly identical regardless of orientation.
Density is the mass per unit volume; specific gravity compares that density to water. Measuring density of irregular specimens can be achieved via water displacement. For minerals, specific gravity is more common: weigh the crystal in air, then in water, and divide the air weight by the water weight. Quartz typically exhibits a specific gravity of 2.6–2.7, whereas diamond ranges from 3.1 to 3.53. Consequently, a diamond of equal size will feel noticeably heavier than quartz.
Both minerals sparkle, but their optical signatures differ. Luster describes how light reflects off a surface: quartz has a vitreous (glassy) luster, while diamond displays an adamantine luster. The refractive index, a more precise metric, measures how light bends when passing from one medium to another. Quartz’s index falls between 1.544 and 1.553; diamond’s is a striking 2.418. A practical field test involves submerging the crystal in oils of known refractive index. Quartz will nearly vanish in vegetable oil (1.47) or wintergreen oil (1.536), whereas a diamond will remain clearly visible.
Hardness, on the Mohs scale, is a direct indicator of a mineral’s resistance to scratching. Quartz scores a 7, whereas diamond tops the scale at 10. A diamond can scratch quartz, but quartz cannot scratch diamond. Even harder materials—topaz (8) and corundum (9)—will scratch quartz but not diamond. When two diamonds are rubbed together, they can indeed scratch one another. Cleavage assessment requires breaking the crystal: diamonds exhibit distinct cleavage planes parallel to each natural face, while quartz shows no true cleavage, though it may display parting along weakened internal planes.
