4^)2 KKPOKT OF NATIONAL MUSEUM, J!HH». 



complex ciTstals having a hardness of ('» and a specific gravity of ^.84. 

 The transparency and brilliaiicy of the stone resembles that of topaz. 

 One hundi'cd parts contain: Phosphoric acid, 55.86; glucina, 19.84; 

 soda, 28.72. 



BROOKITE. 



ARKANSITK. 



Brookite does not readil}' admit of polish, and in consequence has 

 little use as a gem. Its hardness is 5.5 to 6; specific gravity, 4.12 for 

 ])r()okite and 4. OS for arkansite. Brookite includes the hair ])roAvn, yel- 

 loM'ish, reddish, or ru))y I'ed, transparent to translucent kinds having a 

 metallic adamantine luster. Arkansite includes the brilliant, iron- 

 l)lack, opaque kinds. 



CcMuposition: Titanium, 00.98; oxygen, 39.02. 



CARBONATE OF LIME. 



I'HAKI. ( OUAI, MARHLK CAU ITK AKA(iONrTR. 



Carl)onate of lime is juost widely distri))uted in a variety of forms, 

 the varieties depending upon differences in origin, crj'stallization and 

 structural condition, presence of impurities, etc. With the exception 

 of })earl and coral, the many forms find a use more for decorative pur- 

 poses than for personal, adornment. 



Pearl.H are concretions, consisting essentially of carl)onate of lime, 

 found in the shells of certain mollusks. Thev are the result of an 

 abnormal secretory process caused by an irritation of the mantle of 

 the moUusk, resulting from the introduction into the shell of some 

 foreign body, such as a grain of sand. 



Pearls possess a luster peculiar to themselves, which is called pearly 

 or nacreous. This luster may exist on the exterior surface only of the 

 concretion, or the outer surface may be dull and dead in luster while 

 an inner surface may be clear and lustrous. Their specific gravity is 

 2.5 to 2.T; their hardness 2.5 to 3.5. They may be of any shape, 

 and in some instances of considerable size. In color they range from 

 an opaque white, through pink, yellow, salmon, fawn, purple, red, 

 green, brown, ])lue, black, passing through the several shades of these 

 colors, or of almost any color; in addition they may be iridescent. 

 In general, their color and luster will be that of the interior shell sur- 

 face adjacent to which they are formed. 



The beauty and value of pearls is dependent upon their color, tex- 

 ture or " skin," transparency or " water," luster, and form, the most 

 valuable being those Avhich are round or pear-shaped, slightly trans- 

 parent, free from specks or blemishes, and possessing to the highest- 

 degree the characteristic luster. 



Pearls are liable to detcu'iorate with age, contact Avith acids, gases, 

 and vapors, and though various methods are in use for restoring them 



