Robert Haar said something like:
One of the problems with tapcon screws however, is that if the drill
bit encounters a stone aggregate piece, it will not drill through it.
and you're SOOL. Bits will only chew through the matrix between.

Maybe with a regular drill but a hammer drill with an appropriate bit will
go through through the stones. You just need the right tools and
techniques.

Of course, I believe that quartz changes the situation a little.

From: http://www.skidmore.edu/~jthomas/fai...es/rockid.html

Quartz: hardness of 7, usually clear, may be a little grayish, glassy looks
like grains of glass.

Potassium Feldspar: hardness of 5 1/2 - 6 1/2, pink to light gray, two good
cleavages at right angles, opaque, rectangular minerals with good cleavage.

Plagioclase: hardness of 5 1/2 - 6 1/2, gray, two good cleavages at right
angles, striations on one cleavage, opaque, rectangular minerals with good
cleavage and striations.

Clay: aphanitic, soft, usually gray, but can be red or green, looks muddy.

Calcite: hardness of 3, fizzes in acid.

Dolomite: hardness of 3, fizzes in acid when it is powdered. To powder the
mineral, scratch it with a knife, steel nail, or other metal object.

Halite: hardness of 2, tastes salty.

Gypsum: hardness of 2, does not taste salty.



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