DIAMONDS IN ARKANSAS MISER AND ROSS 271 



Crystal No. 6: Colorless; suggestion of a flattened octahedral twin but may 

 be only a distorted form like the others. 



Crystals Nos. 7 and 8 : Yellow ; like crystal No. 5. 



Crystals Nos. 9 and 10 : Colorless ; rounded, flattened, and distorted hexocta- 

 hedrons. 



MINING AND TREATMENT OF DIAMOND-BEARING MATERIAL 



Probably several hundred diamonds have been picked up by 

 miners from the surface of the peridotite areas, especially the Prairie 

 Creek area, but most of the diamonds have been obtained by wash- 

 ing the diamond-bearing material. Different methods of mining 

 and washing have been employed, in part because of the diverse 

 character of the diamond-bearing material which differs not only 

 from one locality to another but also at different depths. 



Much of the decomposed peridotite, including the surficial black 

 soil called " black ground " and also a large part of the underlying 

 " blue ground," " yellow ground," and " green ground," is soft enough 

 to be washed for the recovery of the diamonds without being crushed 

 or weathered first. Most of the black ground is very sticky, like 

 gumbo, when it is wet, so that it disintegrates with some difficulty in 

 the washing plants. Experiments are said to show that when thor- 

 oughly dried and then washed it absorbs water rapidly, swells, and 

 finally slacks to a thin mud. Much of the diamond-bearing mate- 

 rial, especially that of some of the " blue ground " and " green 

 ground," is fairly hard and requires crushing, exposure to weather- 

 ing, or other treatment for the recovery of diamonds. The unaltered 

 peridotite, called " hardebank," and the unaltered peridotite breccia 

 are so hard and tough that they probably can not be treated in any 

 way for the extraction of diamonds except by crushing, which would 

 doubtless fracture some of the diamonds. 



All the mining has been done in shallow open cuts. Some 

 hydraulic mining has been done (pi. 3, fig. 1), but most of the mate- 

 rial mined has been removed by hand and by means of plows and 

 scrapers, and hauled to washing plants in tram cars which have a 

 capacity of 16 cubic feet each. 



In hydraulic mining the water carries the disintegrated diamond- 

 bearing material through a sluice trough, from which the sluiced 

 materials are washed into small plants, where they are sized and 

 jigged. Then the concentrates are placed on smooth sheets of metal 

 and carefully searched for diamonds. 



A log washer was used for a short time in the Ozark washing 

 plant, but it was not successful on accoimt of the stickiness of the 

 " black ground " that was washed in it. 



Washing pans of the type common in the South African diamond 

 fields have been successfully used. Such a pan is circular, has a 



