A Reader's Primer
On the science of the stone.
A crystal is a solid whose atoms are arranged in an orderly, repeating three-dimensional pattern - a lattice. That single fact is responsible for almost every property The Crystal Atlas records: color, hardness, cleavage, refraction, piezoelectricity, and flash.
§ I
The Seven Crystal Systems
Every mineral on earth belongs to one of seven symmetry classes, defined by the axes and angles of its unit cell.
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Cubic
a = b = c · 90°
Salt, pyrite, garnet, fluorite
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Tetragonal
a = b ≠ c · 90°
Zircon, rutile, apophyllite
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Orthorhombic
a ≠ b ≠ c · 90°
Topaz, olivine, aragonite
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Hexagonal
a = b ≠ c · 120° / 90°
Beryl, apatite
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Trigonal
rhombohedral
Quartz, calcite, tourmaline
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Monoclinic
a ≠ b ≠ c · one oblique
Selenite, orthoclase, azurite
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Triclinic
no equal axes
Labradorite, kyanite, turquoise
§ II
Why stones have color
Color is rarely pigment. It usually arises from trace ions substituted into the lattice (Fe³⁺, Cr³⁺, Ti⁴⁺), from color centers created by natural radiation, or from structural effects like thin-film interference - labradorescence, opal fire, adularescence.
§ III
Hardness & cleavage
The Mohs scale (1 talc → 10 diamond) measures scratch resistance, not toughness. Cleavage - the tendency to split along atomic planes - is what makes selenite peel and fluorite cube, regardless of hardness.
§ IV
Piezo- & pyroelectricity
Quartz and tourmaline generate voltage under mechanical or thermal stress. This is not metaphor - it is measured in millivolts. The property that runs your wristwatch is the same one that has drawn dust to warm tourmaline for three centuries.