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PREFACE to THE SEcond EDITION ix
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agate amber angle artificial attractive axis bands Benitoite beryl biaxial blue Brazil brown cabochon called carats Ceylon character cleavage color colorless composition contains corundum crystals dark deposits determined diamond direction dispersion double refraction easily emerald example faces facets figure forms fracture frequently garnet gem minerals glass gold green hardness heat hexagonal imitations important index of refraction indices iron Italy known Lapis lazuli less light liquid localities luster masses material mean metallic method mineral mounted natural observed obtained occurs opal opaque optical parallel pearl pink placed Plate polarized polished positive precious present principal produced properties quartz result rocks rounded ruby sapphire solution sometimes South specific gravity spinel stones structure substances surface synthetic term Titanite topaz tourmaline translucent transparent turquois twins types usually varies varieties various Vitreous weight yellow zircon