QUARTZ. 133 



less ; but sometimes present topaz-yellow, amethystine, rose 

 or smoky tints. Also of all degrees of transparency to 

 opacity, and of various shades of yellow, red, green, blue and 

 brown colors, to black. In some varieties the colors are in 

 bands, stripes, or clouds. H=7. Gr=2*6 — 2*7. 



Composition : quartz is pure silica. Opaque varieties of- 

 ten contain oxyd of iron, clay, chlorite or some other mineral 

 disseminated through them. Alftne before the blowpipe infu- 

 sible, but with soda melts readily with a brisk effervescence, 



Dif. Quartz is a constituent of many rocks, and composes 

 most of the pebbles of the soil or gravel beds. There is no 

 mineral which takes on so many forms and colors, yet 

 none is more easily distinguished. A few simple trials 

 are all that is required. 



1. Hardness — scratches glass with facility* 



2. Infusibility — not melting in any heat obtained with the 

 blowpipe. 



3. Insolubility — not being attacked, like limestone, in any 

 way, by the three acids. 



4. Absence of any thing like cleavage. One variety ap- 

 pears to be laminated, but it consists merely of apposed 

 plates, which are the result of having been formed or de- 

 posited in successive layers, and cannot be mistaken for 

 cleavage plates. 



To these characteristics, its action with soda might be 

 added. In the crystallized varieties, the form alone is suffi- 

 cient to distinguish it. 



Varieties. — The varieties of quartz owe their peculiar- 

 ities either to crystallization, mode of formation, or impuri- 

 ties, and they fall naturally into three series. 



I. The vitreous varieties, distinguished by their glassy 

 fracture. 



II. The chalcedonic varieties, having a subvitreous or 

 a waxy luster, and generally translucent. 



III. The jasyery varieties, having barely a glimmering 

 luster and opaque. 



1. Vitreous Varieties. 

 Rock Crystal. Pure pellucid quartz. 

 This is the mineral to which the word crystal was first 

 applied by the ancients; it is derived from the Greek krus* 



What is said of the color and appearance of quartz 1 How is it dis- 

 tinguished ? What are the three classes of varieties ? What is the 

 origin of the word crystal ? 



• 12 



