102 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



Hardness. — 7 when unaltered, but sometimes less because of de- 

 composition. 



Specific gravity.— 3. 70 ±0.05. 



Optical properties. — -Mean refractive index, 1.75; double refraction 

 weak, 0.01; optically biaxial, positive. 



The mineral occurs in metamorphic rocks, especially mica schist 

 and is usually recognized from the cruciform character of its crys- 

 tals. This peculiar cross or star shape of 

 the crystal renders well-developed speci- 

 mens somewhat prized for ornaments. 

 The crystal faces are usually coated with 

 mica when found, but this can be readily 

 scraped off with a knife and the surfaces 

 can be improved by polishing slightly, and 

 the mineral is usually put on the market 

 in this form. The natural crystals are 

 often ground on the edges to "improve" 

 the cross-like effect. The values are purely 

 nominal. 



LIST OF SPECIMENS. 

 Fig. 11. — Staurolites or "Fairy 



stones." LOCALITY NOT RECORDED. 



Five crosses of various sizes. Gift of H. P. Petersen (fig. 11) No. 1029 



Sunstone. — See under Feldspar. 



THOMSONITE. 



Variety. — Lintonite. 



Composition. — Aluminum, calcium, and sodium hydrous silicate. 



Crystallization. — Orthorhombic, commonly columnar radiated. 



Color. — Variable flesh-red, yellow, green, and white. 



Luster. — Vitreous, inclined to pearly. 



Ha ixlness . — 5.5. 



Specific gravity. — 2.4. 



Optical properties. — Doubly refracting, mean refractive index 1.503. 



The mineral occurs as a secondary product in radiating and con- 

 cretionary forms, filling amygdaloidal cavities in basic lavas. On 

 exposure these lavas sometimes break down and the amygdules are 

 liberated, and in the Lake Superior region often accumulate as 

 pebbles in considerable quantities along the beaches, whence they are 

 gathered to be cut for local souvenirs. The value is little more than 

 cost of cutting. 



LIST OF SPECIMENS. 

 UNITED STATES. 



Minnesota. 

 Grand Marais, Cook County: 



Cabochon, elliptical girdle; mottled, white, red-brown, etc.; 50.92 

 carats; 37 by 24 by 9 mm. Isaac Lea collection No. 1261 



