Richard J Kinch wrote:
Don Bruder writes:
Actually, by my understanding, quartz is indeed a form of silica, but
the reverse isn't necessarily true.

In the case of common sand, it is. The yellow stuff on the beach is quartz
with some iron (oxide). Silica sand is also called quartz sand.

Not so. See, for example, http://en.wikipedia.org/wiki/Silica .
Partial "
The most common constituent of sand in inland continental settings and
non-tropical coastal settings is silica, usually in the form of quartz
because the considerable hardness of this mineral resists erosion.
However, the composition of sand varies according to local rock sources
and conditions.
"

Ted