Gems Notes and Extracts Augusto Castellani, Mrs. John Brogden 1871

8 GEMS.IV. Pearly brightness; that which resembles theparticular light of a pearl.V. Silky brightness; that which gives the appearanceof silk-stuff.The value of a gem depends principallybeauty of its colour. The colouring011 thematerial isgenerally composed of metallic oxides.The wonderful variety of colours in the gems, whichmore than all others resemble those of the solarspectrum, gave rise to the diversityof names whichare often given to the same substance. The redcorundum is the oriental ruby the blue corundum is;the oriental sapphire; the yellow corundum is theoriental topaz. The bluish-green emerald is nowcalled beryl. Quartz has many names; and more orless value in its different colours.There are crystals which not only have a variedgradation of the same colour, but also present threedistinct colours.I possess a quartz in which the central disc is red,surrounded by a green zone, ending ina white band.Corundums have been seen in which shone red, blue andyellow ;and we sometimes see the tormaline variouslycoloured, watered, and marked with strange figures.In some gems the colour differs according to thereflection or transmission of light. The opal withreflected light is prismatic, and with transmitted lightit is whitish. The tormaline with reflected light isred, blue, or some other equally bright colour, butwith transmitted light it is olive.