JACINTH, a name given to the reddish-brown variety of zircon, known also as Hyacinth. The hyacinthus of ancient writers appears to have been our sapphire, or blue corundum, while the jacinth or hyacinth of modern mineralogists may have been the ancient lyncurium. The true jacinth is a silicate of zirconium, crystallizing in the dimetric or tetragonal system, and exhibiting strong double refraction. Its hardness is denoted by 7'5 ; that is to say, it is harder than quartz but not so hard as topaz. The most distinctive feature of the stone, serving to distinguish it from other minerals with which it is likely to be confounded—such as garnet, topaz, and cairngorm—is its high specific gravity; this varies, however, in different varieties of zircon from 4 - 05 to 4 ’ 75. On ignition, most zircons increase in density without loss of weight; but Professor Church has shown that the jacinth of Mudgee, when heated, remains practically unchanged in density, though it loses colour. It is only when the native silicate of zirconium presents a red colour that it is known as jacinth or hyacinth,— those varieties which are of yellow brown and green colours being distinguished, if transparent, by the name of jargoon, while the dull-coloured varieties, more or less opaque, are termed simply zircon. The lustre of the zircon when polished is of the peculiarly brilliant character designated adamantine, and indeed some of the pale jargoons are often sold as inferior diamonds. The Singalese variety, found chiefly at Matura, has been termed “Matura diamond.” The true jacinth, or red zircon, is an extremely rare stone. Fine examples, however, have been found of late years as pebbles among the auriferous detritus at Mudgee in New South Wales. Small crystals occur in the river-sands of ...