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.

The hardest mineral in any abundance is sorted out by the surf to be beach
sand, all softer minerals (such as calcium carbonate, which is much more
abundant) being milled to dust in a sort of competition.

If diamonds existed in any abundance, we'd have diamond beaches instead of
quartz.