506 REPORT OF NATIONAL MUSEUM, 1900. 



Butte, Trinity, and Del Norte counties, California. In 1S56 the 

 Dewey diamond, weighing when cut 11^ carats, was found at Man- 

 chester, near Richmond, Virginia. 



DIASPORE. 



The smallness of the crystals, together with their brittleness, makes 

 this mineral of little use as a gem. Its hardness is 6.5 to 7. Specific 

 gravity, 3.3 to 3.5. Luster, brilliant; pearly on cleavage face. Color, 

 grayish white, greenish gray, hair brown, topaz to fawn 3'ellow, and 

 colorless; occasionally violet blue in onedirection, plum blue in another, 

 and pale asparagus green in a third. Diaspore occurs foliated and 

 massive and in orthorhombic prismatic crystals; usually small, thin, 

 and flattened. The mineral is commonly found with corundum or 

 emery in dolomite, chlorite schist, and other crystalline rocks. Per- 

 haps the finest diaspores are those found near Unionville, Chester 

 County, Pennsylvania. At this locality crystals have been found meas- 

 uring from i to li inches in length and i inch in thickness. 



One hundred parts contain: Alumina, 85.1; water, 14.9. 



DIOPSIDE. 



Diopside is occasionally cut as a gem. It is a variety of pyroxene 

 occurring in monoclinic crystals of a prismatic habit. Its hardness is 

 about 6. Specific gravity, 3.2 to 3.38. Luster,- vitreous. Color, 

 white, of several shades, pale green to dark green and nearly black. A 

 varietj' containing chromium is often of a fine bright-green color. 



Pyroxene is a common mineral in crystalline limestone and dolomite, 

 serpentine, and eruptive rocks; occurs also in granitic rocks and 

 metamorphic schists. The variety diopside occurs commonly in lime- 

 stones and serpentines. A famous locality is on the Mussa Alp, in the 

 Piedmont, where it occurs in veins, traversing serpentine, associated 

 with garnet. A similar locality is at Traversella, in the Tyrol. Fine 

 gem diopside occurs also at Dekalb, New York. 



One hundred parts contain: Silica, 55.6; lime, 25.9; magnesia, 18.5. 

 Iron is usually present in small amounts, and chromium is occasionally 

 observed. 



DIOPTASE. 



ACHIRITE — CONGO EMERALD. 



Dioptase is a silicate of copper crystallizing, commonly in prismatic 

 forms, in the rhombohedral division of the hexagonal system. Its 

 hardness is 5 ; brittle. Specific gravity, 3. 28 to 3, 35. Luster, vitreous. 

 Color, rich emerald green. Transparent to subtransi./^ent. Double 

 refraction, strong. Cleavage, rhombohedral and perfect. Pj'roelectric. 



Dioptase is limited in its use as a gem ]>y its softness and brittleness. 

 It occurs in druses and crystalline aggregates on quartz in seams of a 

 compact limestone near Altyn-Tube, in the Kirghese Steppes, whence 



