silica = SiO2 - ie. silicon oxide. In its most abundant form, this is
simple quartz - ie. sand. (other silca minerals are strange high pressure
or temperature affairs - even in geology classes they're rare. Some are
only formed in meteorite impacts...)
Silica can occur in an amorphous (ie. non-crystalline) state - usually
found as flint or opal.
Both are chemically inert as CP are concerned. Use either, obviously
irridescent opal is probably better used for other things though... :-)
This sounds like the stuff you want. The kind of sand to avoid is
"road grit" or some similar name which is a grit (coarse sand) / salt mix
- commonly available over here, and used for gritting roads.
Garden Centre sand (general purpose or otherwise) should not include salt,
as the majority of plants hate the stuff - not just CP.
I would have thought that sandblaster sand is the same - salt will not
be too good for corrosion.
Richard