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PRECIOUS OPAL* 231 Constituent Parts. Silica, - - 90 Water, - - 10 100 Opal of Czseherwenitza, according to Klaproth, Gcognostic Situation. It occurs in small cotemporaneous veins in clay-pof- phyry, and is generally accompanied with semi-opal. It would appear also to occur in floetz amygdaloid, and in minute portions in veins in gneiss. Geographic Situation. It is found in greatest abundance in clay-porphyry at Czseherwenitza, near Kaschau in Upper Hungary ; spa ringly in floetz amygdaloid in the Faroe Islands; and iu floetz-trap rocks in the north of Ireland, at Sandy Brae. Formerly small portions of it were found in the mines hear Freyberg in Saxony. De Dree mentions that it Occurs also in South America. Uses. The only opal mines in the world are those of Czscher- Wenitza in Hungary, which have been worked for a long time : even so early as towards the end of the fourteenth century, about 300 men were employed. This mineral, () n account of its beauty and rarity, is considered by jewellers as a gem ; and is worked into ring-stones, necklaces, ear-pendants, and other ornaments. It is cut into a convex form, or cn cabochon, as this form shews its I> 4. colours