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Symmetry, 14; planes, 13, 14; axes, 13, 14; centre, 14.
Synthetic and manufactured gems, 56-65.
Synthetic gems, 3; value and uses of, 58-64.
Synthetic rubies and sapphires, 56; properties of, 60, 61-63, 111,
116.
Synthetic spinel, 56, 60, 61, 63, 118, 126. Synthetic stones, 16, 62.
Synthetic topaz, 61. System of crystallization, 67-68.
T
Table, 42, 43, 44, 52, 53, 62, 102.
Tagore, Raja Sourindra Mohun, 5.
Tavernier, 46, 74, 80, 115, 129.
Tawmaw, 148.
Tertiary age, 8, 148.
Tetartohedral, 14.
Tetragonal system, 13, 15.
Tetrahedron, 13, 15.
Tiger's eye, 41, 141, 142.
Topaz,
6, 7, 10, 14, 17, 19, 57, 126, 128, 129; introduction, 126-127; grades,
127-128; uses, 128; occurrence, 128; industry and marketing, 129;
celebrated topazes, 129.
Topazolite, 132.
Total reflection, 31, 32, 33, 45.
Tourmaline,
6, 7, 10, 14, 19, 45, 58, 106, 108, 128, 135; introduction, 133;
physical properties, 134-135; grades, 135; occurrence, 136.
Tourmaline tongs, 134.
Transparency, 17.
Trap cut, 41, 45.
Transparency or Diaphaneity, 18.
Treated gems, 56.
Triassic age, 8.
Triclinic system, 13, 15.
Triplets, 64.
Tully, Refractometer, 34, 35-36, 37.
Turquois, 12, 17, 152-153; Persian, 8.
Turquois matrix, 153.
Turquois mines, 1.
Twin crystals, 16.
Twirdoru, 112.
Twinning in diamonds, 68.
187
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