134 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



6. OCCURRENCE AND ASSOCIATION OF PRECIOUS STONES. 



Under this heading are included exhibits designed to illustrate the 

 occurrence and association in nature of the various stones described 

 in the preceding pages. These are (1) a comparatively small mis- 

 cellaneous collection of specimens from various sources showing the 

 rough gem material embedded in or associated with other minerals 

 as found; (2) a larger collection arranged in what is known as an 

 American case, and comprising the Gardner F. Williams collection 

 illustrating the occurrence of the diamond in South Africa; (3) the 

 granite pegmatites and their associated minerals from the eastern 

 United States and southern California, the last named exhibited in 

 the Geological Hall on the first floor. 



1. MISCELLANEOUS SERIES. 



This comprises two cases in one of which is shown a variet}^ of 

 gem minerals, including both precious and semiprecious stones, either 

 in the matrix or in the rough state as found in nature. The other 

 contains precious opal in the matrix and includes examples from 

 Australia and Mexico, as well as from the more recently discovered 

 field in Humboldt County, Nevada, which furnishes a great variety 

 of colors, ranging from the so-called "black opal" to the pale, 

 iridescent shades. 



2. DIAMOND-BEARING ROCKS OF SOUTH AFRICA. 



The South African collection comprises about 100 specimens of 

 rocks and mineral concentrates typical of the mines, together with 

 illustrations in the form of photographs and engravings. Below is 

 given a transcript of the label accompanying the collection. 



SOUTH AFRICAN DIAMOND MINES. 



[Collections illustrating the occurrence of diamonds in the De Beers Consolidated Mines (Ltd.), Kimberley, 

 South Africa. Gift of Mr. Gardner F. Williams.] 



The country rock immediately below the surface soil is an olivine diabase, locally 

 called basalt (specimen no. 75898). Below this is a dark shale (specimen no. 75902) 

 which is succeeded by a melaphyr (specimen no. 75910), and this by quartzite (speci- 

 men no. 75920). Through all of these have been extruded the diamond -bearing peri- 

 dotite-breccia, shown in its fresh condition in specimen no. 75,933, and in the decom- 

 posed condition known as "blue ground " in specimen no. 75,932. The diamonds are 

 now regarded as having originated through the condensation of metallic carbides in 

 the peridotite while at a great depth below the surface, and to have been brought up 

 in the magma to their present position at the time of its intrusion. In specimen 

 87701 is shown a cavity from which was removed a large crystal. There are also shown 

 a cast of a diamond crystal weighing 363 carats and several small very perfect 

 crystals and rounded bits (bortz) of genuine diamonds. 



