82 



BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



UNITED STATES. 



New York. 



De'Kalb, St. Lawrence County: 



Cabochon, elliptical girdle; yellow-green; 11.7 carats; 18.5 by 13 by 



6 mm No. 571 



Step-brilliant, square girdle; yellow-green; 2.2 carats; 6.5 by 5.5 mm... No. 572 

 Step-brilliant, rectangular girdle; pale green; 1.9 carats; 8by7by5mm. No. 573 

 Brilliant, rectangular girdle; palegreen; 1.56 carats; 7.5 by 6.5 by 5 mm. No. 574 



LOCALITY NOT RECORDED. 



Step-brilliant, rectangular girdle; yellow-green; 1.18 carats; 10.5 by 5 by 

 2.5 mm No. 578 



Fig. 9.— Large amethystine quartz. 



QUARTZ. 



Varieties. — Silica, silicon dioxide, occurs in nature under a great 

 variety of forms, which are usually divided into (1) phenocrystalline 

 or vitreous varieties, (2) the cryptocrystalline varieties, and (3) the 

 amorphous or colloidal varieties. Here are included the glassy, some- 

 times well crystallized forms, known under the names of quartz, rock 

 crystal, amethyst, citrine, and smoky quartz; the massive forms 

 occurring mainly in veins, known as rose, or milky quartz, accord- 

 ing to color; and a few others with local or trade names. Under 

 the cryptocrystalline and amorphous varieties are included those 

 which have, to the naked eye, no evidence of having any crystalline 



