340 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



In recent times the mineral has been found in Japan as water-dear, pale-yellow, or 

 greenish-blue crystals of moderate size. They occur in river gravels at various places, 

 and have been derived from pegmatite veins intersecting granite and gneiss. Many of 

 the crystals are well suited for cutting as gems, and Japanese topaz will probably become 

 of importance commercially. Blue, green, and yellow topaz has been found also in 

 Kamchatka. 



In Africa topaz was found in former times in the same district of Egypt in which 

 emerald occurs, namely, on Jebel Sahara, near the Red Sea. Numerous ancient topaz mines 

 have been rediscovered here, but have been scarcely worked at all in modern times owing to 

 the low price of topaz. Risk Allah is the only place in this region where topaz is mined at 

 the present time. The mineral occurs also in German South- West Africa, sometimes 

 turbid and cloudy, but mostly transparent and water-clear, though not of a quality suitable 

 for cutting. It varies in colour from wine-yellow to brownish-yellow and is rarely 

 distributed. 



Finally, Australia as a topaz locality must be mentioned. The mineral is distributed 

 widely in this continent, and occurs especially in giavels. Colourless, bluish, greenish, and 

 yellow pebbles, the latter very like Brazilian stones, are found associated with cassiterite 

 and diamond in the stanniferous gravels of the granitic region of New England in the 

 north-east corner of New South Wales. The cassiterite (tin-stone) and topaz have both 

 been derived from the granite. Topaz occurs, in a similar manner, in the rivers further south 

 as an associate of diamond, the distribution of which is shown in the map. Fig. 43. The 

 topaz pebbles found here are often beautiful and of considerable size, the largest weighing 

 several ounces. They are colourless or blue, sometimes yellow. Yellow topaz pebbles have 

 been found also in Owen's river in Victoria and at other places. At all these Australian 

 localities topaz is sought for and cut as gems ; this is probably scarcely the case with the 

 topaz accompanying cassiterite in Tasmania, which is of poor quality. 



ZIRCON. 



Although zircon is of less importance and is less frequently cut as a gem than the 

 precious stones hitherto considered, yet it has a certain vogue, the transparent yellowish-red 

 variety, distinguished by the name hyacinth (jacinth), being most used. 



Zircon is a compound of the oxides of silicon and zirconium; that is to say, a compound 

 of silica and zirconia. It contains 23-77 per cent, of silica (SiOg) and 76-23 per cent, of 

 zirconia (ZrOg), a composition corresponding to the chemical formula ZrO^.SiOj. 



The forms taken by the crystals of this mineral belong to the tetragonal system and are 

 usually very simple ; four of the commonest forms are represented in Figs. 68a to d. The 

 crystals are usually short, comparatively thick, and with faces symmetrically developed on 

 all sides ; they are bounded by square prisms and tetragonal octahedra of two orders. 

 Hyacinth, which is practically the only variety used as a gem, scarcely ever occurs in any 

 form other than that shown in Figs. 686 and c, and in Plate I., Figs. 11 and 12. We have 

 here a square prism of the second order, with its edges sometimes truncated by narrow faces 

 of a square prism of the first order (Fig. 68c), and terminated by a tetragonal octahedron, 

 or pyramid, of the first order, so that at each end there is a four-faced pyramidal 



