2. NAMES AND PHYSICAL AND CHEMICAL PROPERTIES OF PRECIOUS 



STONES. 



About 1,200 mineral species are now (1920) known to science, and 

 of these somewhat less than one-tenth, or in round numbers 100, 

 possess the properties of beauty and durability to such an extent 

 that they are of importance, interest, and value as precious and 

 semiprecious stones or gems. The present paper comprises descrip- 

 tions of these minerals, lists of the cut stones of each represented in 

 the collection, and notes on methods of identification, uses, etc. 

 By way of introduction to the descriptive portion, brief definitions of 

 the properties of the minerals and other explanatory notes are given ; 

 no attempt has been made, however, to make this a treatise on min- 

 eralogy, chemistry, or physics, and for further information concern- 

 ing the various features discussed, reference should be made to works 

 on those subjects. 



Names. — The names of minerals have in part come down to us 

 from the ancients, and in part been constructed by adding the suffix 

 ite (from ites, similar to) or lite (from lithos, stone) to an appropriate 

 root. As far as possible the names general^ accepted by mineral- 

 ogists have been adopted here. 



Many stones, in addition to the standard names by which they 

 are known to science, are also called by one or more synonyms, 

 usually popular or trade names; the most important of these are 

 added in each tabular description. 



Chemical composition. — The chemical composition and formulas 

 of the minerals are stated in a simple form. For the benefit of those 

 unfamiliar with chemical terms it may be explained that the majority 

 of mineral substances are compounds of two or more elements, in 

 more or less definite proportions; and that these compounds are 

 named by stating first the names of the elements forming the electro- 

 positive or basic part of the compound, and then those of the ele- 

 ments forming the electro-negative or acidic part. The names of 

 the negative elements are combined, usually in abbreviated form, 

 and provided with significant suffixes, such as ide when a single 

 element is concerned, and ate when oxygen is also present. 



The formulas of the compounds are derived by placing the sym- 

 bols of the elements together, with subscript figures to indicate the 

 number of atoms of each represented. For definitions of the various 

 chemical terms, and additional details, the reader is referred to 

 textbooks of chemistry. The following table shows the elements 

 that may be present in important amounts in precious stones, with 

 their symbols: 



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