GEMS OF AUSTRALIA, 79 



6. The "fracture," when broken, is also a test of some import- 

 ance. Thus, quartz has a conchoidal fracture, like the shell of 

 an oyster ; the topaz breaks across its prism or line of axis, 

 nearly as flat as a piece of plate-glass, and with a high lustre. 

 The fracture of other minerals is said to be splintery, &c. 



7. The degree of polish, or "lustre," is another test for minerals; 

 some being adamantine, as the diamond, — vitreous, as glass, — • 

 metallic, as gold, — resinous, as pitch, — earthy, as chalk, &c. 



8. The " streak", or mark made by a fine file across a mineral 

 capable of being scratched by it, or by a still harder mineral, ia 

 another test, though of minor importance. Por instance, many 

 iron ores (hsematites, &c.) yield a red streak ; or, as the Grerman 

 miners technically express it, es hlutet (it bleeds) ; some, though 

 coloured stones, give a white or colourless streak, as spinelles of 

 all colours ; others, though dull, show a metallic or shining streak, 

 as plumbago, soap-stone, &c. 



9. " Cleavage" is of importance, to ascertain the primitive form 

 of a crystal, by splittuig off parallel faces, or by removing 

 edges or angles, and hence determining the "System" to which it 

 belongs. In some cases also one mineral may be distinguished 

 from another by cleavage. Thus, water-worn or rounded pebbles 

 of white topaz and quartz present the same appearance externally ; 

 but you may cleave the topaz straight across it, in one direction ; 

 whereas the quartz pebble has no distinct cleavage-plane, and 

 would break with a conchoidal or shell-like fracture. 



10. The last and most valuable test, in the case of gems, is 

 their external form or " crystal." No term is more generally mis- 

 applied ; as most people of the present day (not being minera- 

 logists) apply it, as did the Ancients, only to rock-crystal or 

 quartz : the Ancients believing that the latter was water, 

 solidified by enormous pressure; and hence they gave it the name 

 of krustallos, being the Grreek for "ice." 



The best definition of the word " crystal" is given by Professor 

 Ansted. 



In the transition from the gaseous or fluid state to the solid, 

 many substances assume the condition of a regular geometrical 

 solid, bounded by plane faces. (See chart of crystals herewith.) 

 Such are the forms most readily recognized as crystals ; and they 

 occur, whether the solidification takes place by the separation of 

 the solid from an aqueous solution, or by cooling from igneous 

 fusion. The beautiful crystals occasionally found in Nature, and 

 imbedded in various rocks, have doubtless been formed under 

 one of these conditions. 



Each substance usually exhibits a peculiar crystalline form of 

 its own, although frequently much modified or varied ; yet an 

 absolute geometrical identity of form is not expected, even in the 

 same mineral substance. 



