Crystal structure, also referred to as crystal systems or crystallography, refers to the internal structure of minerals. Based on the axes of symmetry, there are seven main categories of crystal structure:

  1. Isometric (or cubic) – has three axes of symmetry which intersect at right angles, all three being equal length
  2. Tetragonal – has three axes of symmetry which intersect at right angles, two being the same length (the third being either longer or shorter)
    • examples include rutile, apophyllite, and zircon
  3. Orthorhombic – has three axes of symmetry which intersect at right angles, all three being different lengths
    • examples include alexandrite, iolite, zoisite, cross stone, tanzanite, topaz
  4. Hexagonal – has four axes of symmetry, three of which are in the same plane, the same length, and have 60° angles (forming a hexagon); the fourth at a right angle to the hexagon and a different length than the other axes
  5. Trigonal – exactly like hexagonal, except that the axes in the same plane are offset such that they form more of a triangle; this group is sometimes still called hexagonal for this reason, and is often considered a subset of the hexagonal group
  6. Monoclinic – has three axes of symmetry where two are at right angles and the third is offset, all three being different lengths
    • examples include azurite, howlite, moonstone, chrysocolla, kunzite, and serpentine
  7. Triclinic – has three axes with no right angles, all three being different lengths
    • examples include amazonite, labradorite, rhodonite, kyanite, and feldspar
  • Amorphous – no lines of symmetry, meaning that it actually has no crystal structure
    • examples include amber, obsidian, opal, and moldavite

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