MINERALOGY. 



Olivex-Ore, or Olive Copper-Ore, arfeniate of copper. 

 See Coi'PEK-Ores. . 



On'yx, a variety of chalcedony with white and grey 

 ftripes. See Chalcedony. 



Opal, Quarts refwUe opalln, Haiiy. ( See Gem. ) This 

 mineral is divided into feven fub-fpecies by Mr. Jamefon. 



Pralous Opal has frequently a milk-white colour inclining 

 to blue, and difplays brilliant and changeab e refleftions ot 

 CTeen, blue, yellow, and red. It is tranilucent or femi- 

 trarfparent, and when placed between the eye and the light 

 exhibits a beautiful yellow or blue colour. It occurs in 

 veins in clay-porphyr)', either maflive, difTemniated, or ni 

 plates ; it has a fhining or fplendent vitreous luftre, a pcrfeft 

 conchoidal frafture, fcratches glafs, and is eaiily frangible. 

 Some varieties adhere to the tongue. Precious opal becomes 

 white and opaque before the blow-pipe, but is infufible. 

 The fpecific gravity is 2.1 1 0- According to Klaproth,the 

 conftituent parts are. 



Silex 

 Water 



90 

 10 



100 



Hence it appears, that the pi-ecious op.il is properly 

 a hydrate of pure filiceous earth. This mineral, on account 

 of its beauty, is employed in jewellery, and is held in great 

 eftimation. Jewels of opal muft be kept with care, as 

 they are eafily fcratched, and are apt to crack on fudden 

 changes of temperature. Precious opal is found more 

 abundantly at Cafcherwenitza, near Kafchau, in Upper 

 Hungary, than in any other known fituation. It occurs 

 there in clay-porphyry, and mines of it have been worked 

 for a long period ; towards the end of the fourteenth 

 century, about 300 men were employed in thefe mines. 

 Precious opal is fometimes fparingly found in the bafaltic 

 rocks, in the north of Ireland, and in the Faroe iflands. 

 The finer varieties of opal are named oriental opal. 

 Tavemer, however, informs us, that no precious opal is 

 found in the eaft, and thofe which are fold as oriental are 

 brought from Hungary. Thofe varieties of opal that 

 adhere to the tongue are lefs tranflucent than the others, 

 and more dull ; but when immerfed in water, they become 

 nearly tranfparent, and acquire a beautiful play of colours. 

 Thefe opals have received various names, as the oculus 

 mundi, hydrophane or changeable opal. They are much 

 prized by collectors. To preferve their beauty, the water 

 in which they are immerfed (hould be perfectly pure, 

 and they (hould be taken out as foon as they have acquired 

 their full tranfparency. If thefe precautions are neglefted, 

 the pores become filled with earthy particles depofited from 

 the water, and the hydrophane will remain more or lefs 

 opaque. 



Common Opal differs from precious opal, principally by 

 its inferior luftre and tranfparency, and the greater variety 

 of its colours, which are either milk-white, grey, yellowilh- 

 white, yellow, red, or green. The fame fpecimen never 

 (hews more than one colour by reflefted light, but the 

 milk-white variety, when held oppofite the light, tranfmits 

 a wine-yellow colour. Common opal occurs maffive, 

 diffeminated, and in (harp angular pieces. Internally the 

 luftre is (hining, fplendent, and vitreous ; the frafture is 

 conchoidal ; it fcratches glafs, and is eafily frangible. Before 

 the blow-pipe it is infufible. The fpecific gravity is 2. 01 

 to 2.14. According to Klaproth, its coaftituent parts 

 are, 



Opal of Koremutz. 



Silex - 98.75 



Alumine - 10 



Oxydoflron 10 

 Water 



98.95 



of Telkobayna. 

 9350 

 1. 00 



5 

 99.50 



Common opal occurs in veins of porphyry and ferpentine, 

 and with chalcedony in bafaltic amygdaloid ; it is found in 

 fome metallic veins in Cornwall. Green commom opal is fome- 

 times cut into ring-ftones. The yellow variety is alfo ufed 

 for jewellery ; it has been called nuax opal and pitch opal. 



Fire Opal, fo called on account of its brilliant red 

 colours and tranfparency, is properly a variety of precious 

 opal : it has hitherto been found only at Zimapan in 

 Mexico. According to Klaproth, its conftitueut parts 

 are, 



Silex - - 92. 



Water - - . 7-75 



Iron ... 25 



Mother-of-Pearl Opal, or Cacholong, is a milk-white 

 variety of opal, not unlike mother-of-pearl. 



Semi-opal. — Its colours are generally darker and more 

 muddy than thofe of common opal ; fometimes feveral 

 colours occur together. Semi-opal is maffive, diffeminated, 

 reniform, and botryoidal ; it has a ghftening lullre, is more 

 or lefs tranflucent, is hard and eafily frangible. The 

 frafture is large and fiat, is conchoidal ; it adheres to 

 the tongue. The fpecific gravity is from 2. to 2.18 ; it is 

 infufible. According to Klaproth, the confl;ituent parts are, 

 Silex ... 81 



Alumine - - 3 ~ 



Oxyd of iron - - 1.7J 



Carbon . . j 



Ammoniacal waters - 8 



Bituminous oil - 0.33 



99.08 



Semi-opal occurs in Scotland and the ifle of Rum, and ia 

 various parts of Europe : it pafTes into chalcedony and con- 

 choidal horn-ftone. 



Wood Opal, Quarz rejinite xyloide, Fr. is petrified wood, 

 penetrated with opal, and is intermediate between common 

 opal and femi-opal. It has a ligneous ftrufture, and is 

 diftinguifhed from wood-ftone by its lighter colours, hi^er 

 luftre, perfeft conchoidal frafture, greater tranfparency, 

 and inferior hardnefs. Wood-opal is cut into plates, and 

 ufed for fnuff-boxes and ornaments. 



Jafper Opal is of various fhades of reddifh-yellow or 

 reddilh-brown ; it is fometimes veined and fpotted ; it has 

 a fhining luftre, between vitreous and refinous, and is 

 opaque or feebly tranflucent at the edges. It is hard, and 

 eafily frangible ; it occurs maffive in large and fmall pieces, 

 in porphyry and in veins. The fpecific gravity is from 

 1.86 to 2.07 ; it is infufible. According to Klaproth, the 

 conftituent parts are, 



Silex . - - 43.5 



Oxyd of Iron • >• 47 



Water - ^ - 7.5 



98 



It 



