230 QtfARTZ FAMILY. play of colour. The colours it throws out are blue, green, yellow, and red. Generally several of these co lours occur in one piece: those specimens are rarer that exhibit but one colour, or where one colour preponderates over the others. The rarest and most beautiful of these colours is the red *. It occurs massive, disseminated, in plates, and in strings or small veins. Internally its lustre is generally splendent, seldom pass ing into shining, and is vitreous. The fracture is perfect conchoidal. The fragments are angular and very sharp edged. It is translucent, and then it exhibits a red and green play of colours; or it passes from translucent into semi transparent, when it exhibits a most beautiful yellow co lour ; or it is semi-transparent, approaching to transpa rent, when the principal colour is azure-blue. It is semi-hard in a high degree. It is brittle. It is uncommonly easily frangible. Some varieties adhere more or less to the tongue. Specific gravity, 2.114, BlumenbacL 2.073, A'arslcn. 2.110, Brisson. Chemical Characters. Before the blowpipe it becomes opaque, and milk- white, but is infusible. Constituent * The play ol colours is caused by numerous minute rent* that traverse this mineral: thin layers of air are contained in them, and these have the property of rellccting the prismatic colours. It is a phenomenon analogsu* to the coloured rings observed by Newton.