66 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



Optical properties. — Mean refractive index, 1.525; double refrac- 

 tion weak, 0.009; optically biaxial, positive. 



Method oj identification.— -Can be readily recognized by its softness. 

 Gypsum occurs in great quantities interstratified with other sedi- 

 mentary rocks, and the pure white varieties are worked under the 

 name alabaster. Only the fibrous form, satin spar, is utilized as 

 gem material and then only in small ornaments as statuettes, beads, 

 etc. VvTiile the luster of the fibrous variety is attractive, its softness 

 precludes its extensive use, and it brings little more than the cost of 

 cutting. 



LIST OF SPECIMENS. 

 GYPSUM, variety SATIN-SPAR. 



ENGLAND. 



Cabochon, elliptical girdle; white; 32.03 carats; 29 by 18 by 12 mm. Gift 



of Clarence S. Bement No. 378 



Bridgeford: 



Two necklaces of 63 beads each, white, 8 mm. diameter No. 377 



HEMATITE. 



Composition. — Iron sesquioxide, Fe 2 3 . 



Crystallization. — Hexagonal (trigonal), rhombohedral. 



Color. — Black; when finely powdered, red. 



Luster. — Metallic; practically opaque. 



Hardness. — 6; fairly durable. 



Specific gravity. — 5. 20 ±0.10. 



Optical properties. — Too opaque for determination of optical prop- 

 erties by ordinary means. 



The mineral can be best distinguished from other black minerals 

 by the color of its powder or its red "streak/' made by rubbing it on 

 unglazed porcelain. Occurs abundantly in beds and in veins and 

 sedimentary rocks, but is too black for use as a precious stone. Is 

 sometimes cut into beads, intaglios, etc., which have a value little 

 more than cost of cutting. 



LIST OP SPECIMENS. 



ENGLAND. 



Two intaglios, tabular, rectangular; black; 17.29 and 17.23 carats; 18 by 15 



by 3 mm No. 1245 



Intaglio, tabular, rectangular; black; 11.67 carats; 16 by 11 by 3 mm No. 1247 



Cumberland: 



Necklace of 48 beads, spherical; black; 12 to 8 mm. diameter No. 1246 



Hiddenite. — See under Spodumene. 



Huntilite. — See under Miscellaneous on page 120. 



Hyacinth. — See under Zircon. 



