PRECIOUS OPAL. 



convex surface, and when large and ex- 

 hibiting its peculiar play of colours in pro- 

 fection, isa gem held in high estimation. 

 Fine stones are, however, extremely rare, 

 and can seldom be obtained the size of a 

 nut. When held between the eye and the 

 light, it appears of a pale red and wine- 

 yellow tint, with a milky transparency. 

 By reflected light, as its position is altered, 

 it displays the most beautiful iridescent 

 colours, particularly verdigris-green, and 

 emerald-green, golden yellow, fire-red, 

 bright blue, rich violet, purple and pearl- 

 grey. All these colours are generally dis- 

 played in the same specimen, being arranged 

 in small spangles, in which case it is called 

 Harlequin Opal, or in broader plates or in 

 waved delineations. Sometimes only one 

 colour is present, in which case the rich 

 orange-yellow (called Golden Opal) or vivid 

 emerald-green are the most beautiful. 



An analysis of a Hungarian specimen 

 (S.G. 2-07) afforded Damour : 



Silica 93-90 



Water .... 6-10 



100-00 

 while another yielded to Von Kobell 10-94 

 per cent, of water. 



Precious Opal is found in irregular nests 

 and veins, disseminated in trachyte at 

 Czervenitza, near Keshan, in Hungar}' ; at 

 Frankfort; Gracias a Dios, in Honduras; 

 also in Mexico; and in small rounded 

 pieces in sand in Ceylon ; and in Iceland. 



The Opal has of late years become much 

 worn as a gem, frequently set surrounded 

 with brilliants. To be fully appreciated, 

 the Opal requires a close inspection, and it 

 is for that reason, probably, that the great 

 admiration in which it is held in eastern 

 countries may be partly accounted for. By 

 the people of most nations gems are ad- 

 mired, to a great extent, in proportion to 

 the brilliancy of their reflections, and the 

 admiration they excite in others rather than 

 for the pleasure they afford to those who 

 wear them ; whereas among eastern nations, 

 where women of rank lead very secluded 

 lives, it is the intrinsic beauty of the stone 

 and the pleasure the wearers derive from 

 beholding it, which chieflj' constitute its 

 value. The finest varieties are termed 

 Oriental Opal. 



Beautiful snuff-boxes and other orna- 

 ments can be made of the porphyry which 

 forms the matrix of the Opal, "when the 

 veins are sufficiently rich in play of colours. 

 In consequence of its softness, coupled with 



PREHNITE. 299 



its extreme brittleness. Opal requires the 

 greatest care in working, a moment of in- 

 attention on the part of the lapidary being 

 sufficient to destroy its beaut3^ 



It is related by Pliny that Marcus Anto- 

 nius proscribed and outlawed Nonius (a 

 Roman senator), on account of an Opal. 

 Rather than part with it Nonius fled, car- 

 rying with him no other property than a 

 ring, in which this Opal was set, which had 

 been valued at 20,000 sesterces (£177,083). 



The largest known Opal is in the Imperial 

 Museum in Vienna. It is 4| inches long, 

 2^ inches thick, and weighs 17 ounces. It 

 has been valued at £70,000. Fine Opals of 

 moderate size have been frequently sold at 

 the price of diamonds of the same size. See 

 Opal. 



Brit. Mus., Case 24. 

 M.P.G. Horse-shoe Case, Nos. 767, 

 770, 771. 



Precious Stone. See Gem. 



Pkedazzite, Petzholdt. A variety of 

 Bitter Spar mixed with Brucite, with a 

 granular structure, a white colour, and a, 

 vitreous lustre on the planes of cleavage. 

 H. 3-5. S.G. 2-634. 



Comp. Ca C + Mg H. 



Analysis, by Roth : 



Lime 33-53 



Magnesia . . . . 23 27 

 Carbonic acid . . .27-45 

 Alumina and peroxide of 

 iron .... 2-88 



Silica 3-28 



Water 10-26 



100-67 

 Locality, Forms mountain masses at 

 Predazzo, in the Southern Tyrol. 



Prehnite, Werner, Haiiy. Rhombic : pri- 

 mary form a right rhombic prism. Occurs 



Fig. 341. 



Fig. 342. 



Fig. 343. 



in crystals which are generally closely 

 aggregated ; also massive. Colour greenish- 

 white, also 3'ellowish-grey or yellowish- 

 green. Translucent to transparent. Lustre 

 vitreous or pearly. Streak white. Brittle. 

 Fracture uneven. Pyroelectric. H. 6 to 6-5. 

 S.G. 2-926., 



