145 



TAB. CCCLXXVII. 



SI LEX quartzum, var. opalinum. 

 Opal. 



I have given (Brit. Min. Ill) a figure and description 

 of the Precious Opal of Cornwall. I now show a variety 

 from Sandy Bras, brought from thence by Humphry 

 Davy, Esq. of high chemical celebrity. Opals have been 

 divided with some difficulty into several kinds; as The 

 Precious Opal, The Common Opal, Semi- or Halb-Opal,, 

 and Wood Opal. The good and bad might have been 

 sufficient distinctions, as the several kinds run into each 

 other. The three first arrange according to their good- 

 ness ; the Precious Opal having a pearly brilliancy with a 

 fine soft display of most of the prismatic colours, and a 

 brilliant lustre peculiar to itself: the Common Opal has 

 less of this lustre ; and the Halb-Opal borders on it so feebly 

 as to be considered as only partly an Opal : the Wood Opal 

 may have all these varieties, but is infiltrated into the re- 

 mains of wood retaining its structure. These distinctions 

 therefore do not all belong to the substance, and become 

 incorrect, or mislead. The present specimens are remark- 

 able : the upper one, for a fine somewhat resinous appear- 

 ance, from a light dull greenish yellow to a resin brown, 

 with a fiery or orange yellow sparkling splendency reflected 

 as the light passes among the flaws. 



The upper specimen is in a sort of decomposing Porphyry, 

 spotted and veined with resin-like Opal mostly dull in colour. 



The lower figure has something of the appearance of light 



VOL. IV. M 



