blue ground, noun phrase

Origin:
See quotation 1920.
Geology, Diamond-mining
Eruptive diamondiferous rock which occurs in vertical cylindrical pipes (see pipe), often having a diameter of several hundred feet, and of unknown depth; blue noun sense 1; kimberlite. See also griquaite, hardebank.
Note:
Blue ground is more compacted and often darker in colour than the looser, weathered layers of yellow ground (see yellow sense b) above it.
1882 C. Du Val With Show through Sn Afr. I. 82Digging, delving, and filling up with the precious ‘blue ground’ huge iron buckets.
1886 J. Noble Cape of G.H.: Off. Handbk 192The ‘blue ground’..far from being barren of diamonds..yielded even better returns than the upper layers of ‘yellow ground’.
1900 H.C. Notcutt How Kimberley Was Held 31The great ten-foot tunnel that stretches right away through the rock from the shaft to the blue-ground.
1905 G.F. Williams in Flint & Gilchrist Science in S. Afr. 320In the case of the Kimberley mines, the diamond-bearing rock or blue ground has been forced up through the geological strata...This rock was described by Professor Henry Carvill Lewis as ‘a phorphyritic volcanic peridotite of basaltic structure,’ which he named kimberlite — a name now generally accepted by geologists.
1911 E. Prov. Herald 11 Dec.They were now sinking Kimberley mine another thousand feet and to the surprise of many croakers they had struck blue ground at the 3,500 feet level under granite.
1920 R.H. Lindsey-Renton Diary (1979) 36The diamonds are found in what is known as ‘blue ground’, the soil being of a colour which might by a stretch of imagination be called pale blue. This soil is found in ‘pipes’.
1931 G. Beet Grand Old Days 98Blue ground was conveyed from the mines and spread out on large open spaces,..allowed to disintegrate by exposure to the elements..over a period of from six to eighteen months.
1965 D. Rooke Diamond Jo 169When they reached the blue ground, they abandoned the claims, thinking that they had reached the end of the diamondiferous ground.
1971 G.S. Switzer in Nat. Geog. Mag. Dec. 864The material in which diamonds are found is a dark, basic rock called kimberlite, after the famous mines in the vicinity of Kimberley, South Africa; miners call it ‘blue ground’.
1983 Motorist 1st Quarter 13The area surrounding Kimberley is mainly blueground owned by De Beers Consolidated Mines or privately.
1985 A.J.A. Janse in Glover & Harris Kimberlite Occurrence & Origin 23When diggers found that much harder compact blue ground underlay the yellow ground, many sold out because they thought that they had reached the bottom of the depression and thus the end of the diamondiferous ground.
Eruptive diamondiferous rock which occurs in vertical cylindrical pipes (see pipe), often having a diameter of several hundred feet, and of unknown depth; bluenoun1; kimberlite.
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18821985