Robert Flory wrote:

Check you garden supply or local rockery... "Oakley Stone" is a nice thin
quartz.mica gneiss that has worked for me. Of course at the time I lived in
southern Idaho where it is found. However, I see it here in California and
have seen it in El Paso Texas.

Bob

Thanks for the suggestions, Bob. Going about to garden supplies now,
there are two quarries here that I know of. Will keep Oakley stone in
mind. Around here, bluestone is the commonest rock. Don't know if it
is calciferous or not (my tap water is pretty much without buffer, pH
7.0, GH 2 deg. KH maybe 1 deg.), but it may not be thin enough. At
this point, folks here have given me plenty to think about and explore.

James