266 . ONOFRITE. 



Alumina .... 30-88 

 Magnesia , , „ .3-82 

 Potash .... 6-38 



Protoxide of iron . . O'SO 



Water ..... 4-60 



101-90 

 BB swells up and fuses to a white blebby 

 glass. 



Soluble in sulphuric, but not in muriatic 

 acid. 



Locality. Posseggen, near Tamsweg in 

 Salzburg, disseminated in slightly micaceous 

 dolomite. 

 Brit. Mus., Case 26. 



Onofrite, Haidinger. Occurs massive 

 with a compact granular texture, and no 

 cleavage. Colour steel grey to blackish- 

 lead-grey, resembling that of Grey Copper. 

 Lustre metallic. Streak shining. PL 2-5. 

 aC. 7-1 to 7-37. 



Comp. Selenide o^ mercury, or Hg, Se = 

 quicksilver 71-7, selenium 28-3 = 100. 

 Analysis, from Clausthal, by Kerl ; 

 Selenium . . . " . 24-05 

 Quicksilver . . . .72-26 

 Quartz .... 2-86 



Sulphur .... 0-12 



99-74 



BB on charcoal gives off the odour of 

 selenium; with soda yields metallic mer- 

 cur}^ 



Localities. Near San Onofve in Mexico, 

 with other ores of mccury. Clausthal and 

 Zorge in the Harz, with Copper Pyrites. 



Brit. Mus., Case 4. 



Onyx. A chalcedonic variety of Quartz, 

 resembling Agate, and composed of alter- 

 nate parallel layers of different colours — 

 usuall}'- of a clear light brown, and an 

 opaque white. Those with alternate bands 

 of white and red and green and white are 

 held in great esteem. 



The Onyx is mentioned in Exodus, chap. 

 xxviii. 17—20, amongst the twelve pre- 

 cious stones which were set on the breast- 

 plate of the high-priest, each one of which 

 was engraved with the name of one of the 

 sons of Jacob. Being the eleventh stone, 

 it bore, in consequence, the name of 

 Joseph. The two stones, also, set in ouches 

 of gold, and placed on the shoulders of the 

 ephod or short sleeveless cloak of the high- 

 priest, with the names of the children of 

 Israel engraved upon them : six of their 

 names on one stone and the other six 

 names on the other stone, according to their 

 birth, were likewise ordered to be Onyx. 



OPAL. " 



See also Genesis ii. 12 ; Exodus xxxix, 6 ; 

 and Job xxviii. 16. 



The Onyx was highly prized by th3 

 ancients, who took advantage of the differ- 

 ence in colour to engrave it into what 

 are called cameos. In these the figure in 

 relief is carved out of one layer of the stone, 

 generally the paler of the two, while the 

 darker part forms the back ground. These 

 fetch very high prices even at the present 

 day, and are highly prized, not for their 

 rarity only, but for their beauty as works of 

 art. See Onicolo, also Sard. 



The tints of the Onyx may be heightened 

 by boiling it for several days in honey and 

 water, and then soaking it in sulphuric acid 

 to bring out the black and white layers ; or 

 in nitric acid, to heighten the colours of the 

 red and white layers. 



Localities. — Onyx is found in Perthshire, 

 in the Isle of Skye ; in Antrim, at the 

 Giant's Causeway, in amygdaloid ; and in 

 Rathlin Island, and on the shore of Lough 

 Neagh. 



Foreign. Oberstein in Saxony. The De- 

 serts of the Nogai Khirghiz in the S. of 

 Eussia. Yemen in Arabia. Guzerat. 



Name. The word Onyx is derived from 

 owl, a nail; " because it hath a white in it 

 resembling that in the nail of a man's 

 finger." {Pliny.) 



Brit. Mus., Case 22. 



M. P. G. Horse-shoe Case, Nos. 580 1 o 

 582, 589. 



OosiTE. A mineral related to Pinite, 

 found near Baden. 



Brit. Mus., Case 32. 



Opal. A kind of resinous or uncleavable 

 Quartz. Occurs amorphous, massive, some- 

 times small-reniform, stalactitic, or large 

 tuberose. Colour milk-white, red -brown, 

 green, and pearl-grey ; generally pale, but 

 sometimes dark, owing to the admixture of 

 other substances. Lustre vitreous, often 

 inclining to resinous and sometimes to 

 pearly. Some varieties exhibit a beautiful 

 play of colours, or different colours by re- 

 fracted and reflected light, which are the 

 more beautiful owing to their not being the 

 result of any particular tint in the stone 

 itself, but from their being produced by re- 

 flected light. Semi-transparent or pellucid . 

 Streak white. Very brittle. Fracture con- 

 choidaL H. 5-5 to 6-5. S.G. l-9to 2-3. 



The play of colours in the Opal is not 

 satisfactorily accounted for. By Brewster 

 it is attributed to the presence of internal 

 fissures and cracks of a uniform shape. 



