Greenstone

• Webster's Revised Unabridged Dictionary
  • n Greenstone grēn"stōn` (Geol) A name formerly applied rather loosely to certain dark-colored igneous rocks, including diorite, diabase, etc.
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• • n greenstone Any one of various rocks, of eruptive origin, in general older than the Tertiary, crystalline-granular in texture, and of a dark-greenish color. The essential ingredients of the rocks formerly classed under the name of greenstone are tri-clinic feldspar and hornblende, with which are associated various other minerals in greater or less quantity, and especially chlorite, mica, magnetite, and apatite. The name is abandoned by some lithologists, but retained by many geologists as a convenient designation for those older eruptive rocks which have undergone so much alteration that their original character is in a measure lost, and cannot be made out except with the aid of the microscope, and not always with that help. The most important of these changes seems to be that the original augite has been converted into hornblende, while a still more advanced stage of alteration is indicated by the presence of chlorite, mica, and other minerals, the predominating color of which is greenish, and to this peculiarity the rock owes its name. While there can be little doubt that many of the so-called greenstones, or melaphyres and diorites, as rocks of this class have of later years been often designated, are altered basalts, there is far from being a general agreement among lithologists as to the proper limitation of these names. See basalt, diorite, melaphyre, trap.
  • n greenstone A very hard and close-textured stone used for putting the last edge on lancets and other delicate surgical instruments, etc.
  • n greenstone A name in New Zealand for several varieties of jade, specifically for pounamu or nephrite, found chiefly on the west coast of the Middle Island: formerly much used by the Maoris for weapons, implements, and ornaments.
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• • Greenstone a rock term, now disused, for any dark-green basic crystalline (trap-rock)
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