OPAL. 

 Haiiy ascribes it to thin films of air filling 

 cavities in the interior of the stone, and re- 

 flecting the light agreeably with the law of 

 Newton's coloured rings; Mohs, however, 

 considers that these would produce irides- 

 cence only. 



Comp. Amorphous silica or Si, frequently 

 accompanied hy small quantities of potash, 

 soda, lime, magnesia, alumina, and peroxide 

 of iron, together with between 5 and 10 per 

 cent, of water. The water seems to be 

 only mechanically mixed with the silica, 

 and varies much in the different kinds of 

 Opal. 



BB infusible, but gives out water and 

 becomes opaque : some varieties, containing 

 iron, turn red. 



Localities. Opal occurs in short irregu- 

 lar veins in porphyry, and in vesicular cavi- 

 ties of amygdaloid. The chief localities 

 are Hungary, Mexico, the Faroe Islands, and 

 Iceland. The Opals exported from Hondu- 

 ras are obtained at the mines of the depart- 

 ment of Gracias, which lies between that 

 of Comayagua and the Republic of Guate- 

 mala. 



Name. From opalus, or opalum, the an- 

 cient Latin name for the stone. 



For varieties of Opal, see Precious Opal ; 

 Fire-Opal, or Girasol; Common Opal; 

 Semi-opal ; Hydrophane ; Cacholong ; 

 Hyalite, or Muller's Glass ; Menilite ; 

 Opal Jasper; Michaelite; Alumocal- 

 cite ; Eandajstite ; &c. 



Brit. Mus., Case 24. 



M.P.G. Horse-shoe Case, Nos, 767 to 771. 



Opal, or Opalime Allophane. See 

 Schrotterite. 



Opal Jasper. A kind of Opal contain- 

 ing several per cent, of iron. 



Opale de Feu. See Fire-Opal. 



Opale Terreuse. See Geyserite. 



Operment, Haidinger. See Orpiment. 



Ophiolite (from oi?'?, a snake, and A/^u?, 

 stone). A name given to include the various 

 serpentine rocks. The purer Serpentines 

 are called by T. Sterry Hunt Normal Ophio- 

 lite, and other varieties Calcareous, Dolo- 

 mitic, or Magnesitic, according to the nature 

 of the foreign admixtures which may be 

 intimately mixed with them. 



Ophite (from o(?/?, a snake, because of its 

 spotted appearance like the skin of a snake). 

 See Serpentine. 



Opsimose, Beurlant. Partially altered 

 Fowlerite (crystallized Rhodonite), from 

 Franklin, New Jersey, and Klaperade, 

 Dalecarlia. 



ORIENTAL ALABASTER. 267 

 Or Blanc. Gold Amalgam. See Elec- 



TRUM. 



Or de Chat. A name sometimes given 

 to the small spangles of lamellar Mica as 

 they occur in granitic rocks. 



Or Natif. Native Gold. See Gold. 



Obangite, Bergemann. A name for 

 the orange- vellow coloured varieties of 

 Thorite. S.G. 5-34. 



Oravitzite, Diifrenoy. Orawitzite. 

 A variety of Halloysite from Oravicza in 

 S.E. Hungary. 



Oriental. See Gem. 



Oriental Alabaster (Alabdtre cal- 

 caire oriental, Fr.). The term Alabaster 

 (Albdtre) is applied by the French both to 

 the true mineral of that name and to the 

 Oriental stone, although the two differ 

 widely both in composition and relative 

 hardness. True Alabaster is a sulphate of 

 lime, but Oriental Alabaster is a stalagmi- 

 tic carbonate of lime, which has been de- 

 posited from water holding that substance in 

 solution. Such water, finding its way into 

 the hollows and crevices of rocks, falls down, 

 and on evaporation leaves a calcareous 

 layer, which in time accumulates to such an 

 extent as to entirely fill some cav^erns. 

 Oriental Alabaster is generally clouded and 

 zoned in a concentric manner ; it effervesces 

 with acids, and cannot be scratched with the 

 nail; while ordinary Alabaster is more uni- 

 form in texture, is not acted on by acids, 

 and is so soft as readil}' to yield to the nail. 

 Very fine slabs of Oriental Alabaster are 

 procured from the Pyrennees, Chili, and the 

 mountain of Ourakan, E. of Benisouf in High 

 Egypt. There are also old quarries in the 

 province of Oran in Algeria. This stone wa3 

 held in great estimation by the ancients, 

 who procured it from the neighbourhood of 

 Thebes, from quarries Avhich are still un- 

 exhausted. By the Greeks it was called 

 Alabastrites, and, also, sometimes Onyx, in 

 allusion to its parallel undulating layers, 

 sometimes of different colours, and sup- 

 posed to bear a fanciful resemblance to the 

 markings on the human nail; thus it is 

 alluded to by Dioscorides, as >jQo? aAajffats-T?/- 

 T'/;? b xocXoufj-ivo; owl ; by the Latins, marmor 

 onychites, from its use in making boxes 

 for preserving precious ointments, which 

 boxes were commonly called Onyxes and 

 Alabasters, after the material of which they 

 were made In Matt. xxxi. 7 we read: 

 "There came unto him a woman having 

 an alabaster-box of very precious ointment, 

 and poured it on his head as he sat at 

 meat." (See also Mark xiv. 3, and Luke 



