The dichroiscope is inexpensive, costing but a few dollars, and is very useful for rapidly deciding the species of many stones. The following is a partial list of doubly refracting stones and their twin colors. ───────────────────────┬────────────────────────────────── NAME OF STONE. │ TWIN COLORS. ───────────────────────┼─────────────────┬──────────────── Sapphire (blue) │ Greenish straw │ Blue Ruby (red) │ Aurora red │ Carmine red Tourmaline (red) │ Salmon │ Rose pink " (brownish red) │ Umber brown │ Columbine pink " (brown) │ Orange brown │ Greenish yellow " (green) │ Pistachio green │ Bluish green " (blue) │ Greenish gray │ Indigo blue Emerald (green) │ Yellowish green │ Bluish green Topaz (sherry) │ Straw yellow │ Rose pink Peridot (pistachio) │ Brown yellow │ Sea green Aquamarine (sea green) │ Straw white │ Gray blue Beryl (pale blue) │ Sea green │ Azure blue Chrysoberyl (yellow) │ Golden brown │ Greenish yellow Iolite (lavender) │ Pale buff │ Indigo blue Amethyst (purple) │ Reddish purple │ Bluish purple ───────────────────────┴─────────────────┴──────────────── COLORS. The following is a partial list of the colors of precious stones: _Shades of White._—Quartz, opal, chalcedony. _Shades of Gray._—Labrador, smoky topaz, chalcedony, zircon. _Black._—Obsidian, tourmaline, jet. _Shades of Blue._—Lapis-lazuli, amethyst, chalcedony, spinel, zircon, sapphire, cyanite, tourmaline, turquois, odontolite, fluor spar. _Shades of Green._—Amazon stone, turquois, prase, beryl, blood-stone, epidote, emerald, malachite, chrysoprase, chrysolite, idocrase, olivine, garnet, chrysoberyl. _Shades of Yellow._—Opal, amber, topaz, beryl, jasper. _Shades of Red._—Garnet, carnelian, chalcedony, rose quartz, corundum, tourmaline, spinel, ruby. _Shades of Brown._—Zircon, garnet, smoky topaz, axinite, jasper. _Colorless._—Diamond, sapphire, spinel, zircon, topaz, rock crystal, moonstone. LUSTRE. Well polished precious stones display a decided lustre, which assists in determining their species. The following is a list of some precious stones and their lustre: _Adamantine._—Diamond, zircon. _Resinous._—Garnet. _Vitreous._—Emerald, ruby, spinel. _Waxy._—Turquois. _Pearly._—Moonstone, opal. _Silky._—Crocidolite, quartz cat’s-eye. _Metallic._—Hematite. _Greasy._—Olivine. Some stones vary in lustre, from vitreous to pearly, etc. STREAK. The streak of a mineral is the color of its powder. This powder varies in color, and may be white, gray, red, etc. It is obtained by scratching the mineral with a sharp file, or by rubbing the mineral on the back of an unglazed porcelain plate, when the color of the powder will appear on the plate.