QUARTZ. 



437 



4. Chalcedony, sometimes termed cornelian ; used for seals, etc. 



5. Flint : much used in potteries. " Flint and steel" have been superseded by phosphorus 



6. Hornstone : something like flint, resembling horn. 



7. Jasper : of various colours ; opaque and dull in appearance. 



8. Silicious slate : a combination ; used as a whetstone. 



9. Agate : a mixture of quartz, amethyst, jasper, and cornelian ; very ornamental. 



10. Opal: a peculiar variety, containing water. It is not found in the form of crystal, 

 but in vitreous masses. Its changeableness of hue is proverbial. The "noble" opal is 

 much prized. 



11. Smoky quartz, or cairngorm. 



12. Onyx and Sardonyx. 



Fig. 459. Quartz crystals in various forms. 



We now arrive at some minerals which contain metals. 



POTASSIUM. This metal is so frequently combined in minerals with 

 alumina that we may refer to it with the latter in sequel. There are two 

 natural potassa salts nitre, and sulphate of potassa. Nitre is known as 

 saltpetre, and is of great use in medicine. It is the chief ingredient in the 

 composition of gunpowder. 



SODIUM. We have a number of minerals in this group viz., nitrate of 

 soda (nitratine), which occurs in large quantities in Peru ; rock salt, chloride 

 of sodium, known as salt. It crystallizes in the cubic system. Colour 

 usually white, but it occurs in secondary rocks in company with gypsum, 

 etc. It is sometimes of a reddish colour, or even green and yellow. Biborate 

 of soda is borax, and is found in and on the borders of a Thibetian lake. There 

 are several other combinations with soda : the sulphates of soda 

 viz., thenardite and glauberite, anhydrous and hydrated re- 

 spectively, carbonate of soda, and so on. 



AMMONIA combinations occur in lava fissures, and are not 

 often met with in consequence of their volatile nature. 



CALCIUM. This forms an important group of the minerals, y . 

 which are very white in colour, and not very hard in substance. ay** pa 

 Calcium is the metallic basis of lime. Fluoride of calcium, known as 

 Jluor-spar, most frequently crystallizes in cubes in the first system. Anhydrite 



