Crystals
The Flowers of the
Mineral Kingdom
When you hold a perfect crystal, you can literally see its atomic structure. However, perfect crystals—like perfect people—are rare.
Crystals usually crystallize into one of seven geometric forms: triangles, squares, rectangles, hexagons, rhomboids, parallelograms, or trapeziums.
• Triangles. Crystals with three perfect sides are trigonal crystals.
• Squares. Crystals with four equal sides, like a cube, are isometric crystals.
• Rectangles. Crystals shaped like long, tall rectangles are tetragonal crystals.
• Hexagons. Crystals with six sides are hexagonal crystals.
• Rhomboids. Crystals shaped like short, wide rectangles are orthorhombic crystals.
• Parallelograms. Crystals that are short and stubby with tilted faces at each end are monoclinic crystals.
• Trapeziums. Crystals that look like pyramids with their tops cut off are trapeziums.
Don’t Judge a Crystal by Its Cover
You can’t always tell what kind of crystal you hold in your hand just based on its appearance. Mineralogists who want to identify a crystal typically have to test a number of physical properties, including color, streak, transparency, luster, hardness, cleavage, fracture, specific gravity, and crystal form.
Color. Color can be a crystal’s most obvious property, but it is an unreliable tool for determining what type of crystal you’re looking at. Some minerals are always the same color—but don’t count on it. Just a trace element of another mineral can change its color completely. Quartz, for example, comes in every hue.
Streak. If you rub a crystal firmly across an unglazed white porcelain tile, it will leave a streak—a powdery line. Every type of crystal will always leave the same streak, even if individual specimens have impurities.
Hardness. In 1812, a German mineralogist named Fredrich Mohs devised the Mohs scale of mineral hardness. That scale is now the universal measure of mineral hardness: talc is the softest mineral, followed by gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, and diamond.
Cleavage. If you were to hit a mineral sample with a hammer, it would break along the weakest planes of its crystalline structure—its cleavages. Some minerals break in one direction, while others break in two or more directions. Some minerals cleave perfectly in one direction but poorly in others. Cleavage can be an important clue to a crystal’s identity.
Fracture. Not all minerals cleave easily. Some fracture instead. Unlike cleavages—which are usually clean, flat breaks—fractures can be smoothly curved, irregular, uneven, jagged, or splintery.
Specific gravity is calculated by comparing a crystal’s weight to the weight of an equal volume of water. Heavier minerals have a greater specific gravity.