grinder wheels, elitism?

[VisualThinker7@zzzzzzz]

Diabase is a fine-grained intrusive igneous rock of a composition  similar to
_basalt_ ( http://www.geocities.com/unforbidden_geology/basalt.html ) , but  is
slightly more coarse-grained than basalt. The rock consists of the minerals
calc-plagioclase (bytownite to labradorite) and pyroxene (commonly augite,
pigeonite, and hypersthene) with apatite, magnetite, and olivine commonly
present. Modal rock composition of mafic minerals is less than 90% of the rock  by
volume. Diabase is associated with sill and dike intrusions in the hypabyssal
(upper few km of the Earth's crust). The term diabase is the North American
word  for dolerite, which is commonly used in Europe to describe the same rock
type  (in Europe the term diabase sometimes refers to a weakly metamorphosed
dolerite). The diabase deposits of Egypt can also be called coarse-grained
basalts because of their fine grain size.
Diabase was used by the ancient Egyptians in ways  similar to _basalt_
( http://www.geocities.com/unforbidden_geology/basalt.html ) .  However, its main use
was as pounders for the working of hardrocks. Pounders can  be seen in
hardrock quarries as roughly spherical masses and are found in large  quantities in
the ancient _granite_
( http://www.geocities.com/unforbidden_geology/granite.html )  and  _granodiorite_
( http://www.geocities.com/unforbidden_geology/diorite.html )   quarries at Aswan, and at the _siliceous  sandstone_
( http://www.geocities.com/unforbidden_geology/siliceous_sandstone.html )  quarry at Gebel Ahmar,
near Cairo (Lucas and Harris, 1962). The  spherical diabase pounders observed
in the quarries are similar to cobble to  bolder sized naturally occurring
sediments present in the cataract regions of  the Nile and in the Eastern
desert, presumable the result of erosion of diabase  and rounding as the coarse
sediments were transported down the Nile during  floods.
The reason why diabase was used for pounders by  the ancient Egyptians was
due to its very high fracture strength. As presented  in _Table  2_
( http://www.geocities.com/unforbidden_geology/rock_properties.htm#Table_2)  of the section
on _Rock  Properties_
( http://www.geocities.com/unforbidden_geology/rock_properties.htm )  a typical diabase has an unconfined compressive strength that can
 be considerably higher than that of a typical granite or other
coarse-grained  igneous rock. This means that a solid diabase sphere is usually more
durable  than that of a granite block, and when pounded against granite, the granite
will  mechanically breakdown faster than that of the diabase.