ZIRCON 343 



The dichroism of hyacinth, as of all varieties of zircon, is very feeble ; it is almost 

 impossible to detect any difference between the two images seen in the dichroscope. This 

 instrument, which is frequently so useful in discriminating gems, is therefore of no assistance 

 whatever in distinguishing the feebly dichroic hyacinth from hessonite, which has no 

 dichroism at all. The dichroism of zircon of other colours, though stronger than that of 

 hyacinth, is feebler than that of any other coloured doubly refracting precious stone. 



A characteristic feature of many zircons, discovered by Prof. A. H. Church, is the 

 presence of black absorption bands in the spectrum of white light which has traversed the 

 stone. These absorption bands are attributed to the presence of small quantities of m-anium 

 compounds. 



Some few characters of zircon still remain to be mentioned. It is infusible before the 

 blowpipe, and is not attacked by acids, not even by hydrofluoric acid. When rubbed it 

 becomes slightly electrified, but the charge is not sufficient to render this character of use 

 for purposes of determination. 



Zircon occurs in the older crystalline silicate-rocks, such as granite, gneiss, and other 

 similar rocks of mountainous districts. Opaque zircon is a common constituent of such 

 rocks, and precious zircon, including hyacinth, is found under similar conditions. The 

 crystals are, as a rule, embedded in the rock, and are only rarely found attached to the walls 

 of drusy cavities. Some few of these rocks contain such an amount of common zircon that 

 the name of the mineral is used as a distinguishing prefix, as in the zircon-syenite of the 

 neighbourhood of Fredriksvarn and Laurvik in the south of Norway ; while some other 

 localities, notably in North America, yield common zircon by the hundredweight. Isolated 

 crystals of zircon of the hyacinth variety are sometimes found embedded in the younger 

 volcanic rocks ; for example, in the basalt at Expailly near Le-Puy-en-Velay (Department 

 Haute-Loire) in France ; in Germany in the so-called mill-stone lava of Niedermendig on 

 the Laacher See, in the basalt of Unkel on the Rhine, and in some basalts in the Siebengebirge. 

 A crystal of hyacinth, partly freed from the black basalt in which it is embedded, is shown 

 in Plate I., Fig. 12. It is improbable that such zircon crystals were actually formed in the 

 basaltic rock ; their presence there may be explained by supposing that fragments of granite, 

 or rock of similar nature containing these crystals, were caught up by the glowing, fluid 

 basalt magma and all their constituents melted down except the resisting hyacinth, which 

 i-emained unaltered, thus becoming an apparently normal constituent of the basalt. 



Crystals of zircon are set free by the weathering of the mother-rock, carried away with 

 the rock debris by running streams, and thus eventually become a constituent of sands and 

 gravels. Owing to the unalterable nature of their substance, such crystals remain perfectly 

 fresh and unweathered. The zircon used for cutting as gems is derived exclusively from 

 such sands and gravels, never from the solid rock. 



Various localities in the island of Ceylon send to the markets of the world almost the 

 whole of the supply of hyacinth and other gem varieties of zircon. They are quite 

 abundant, and are collected from the gem-gravels, together with spinel, sapphire, cafs-eye, 

 and other Cingalese gems. The principal sources are the deposits in the neighbourhood of 

 the town of Ratnapura in the SafFragam district, and those of Matura in the south of the 

 island (Fig. 59) ; from the latter place come the colourless and pale-coloured, ignited 

 hyacinths known as " Matura diamonds." 



The crystalline form of the zircons found in these sands is still recognisable, although 

 the crystals, like those of the other precious stones with which they are found, are much 

 rounded. The crystals of hyacinth found in Ceylon are, as a rule, rather small, at most no 

 larger than a lentil ; specimens the size of a pea are rare, and still larger stones quite 



