Oyster shell (chicken grit) is an excellent source of calcium for eriostemmas and imperialis, which seem to have higher calcium needs.

It should not be used as a medium, though.

Also if you plant to use crushed oyster shells,make sure you boil the shells,to leach out all the salts, which could be bad for the roots. I save egg shells and let them dry out for a week, then grind them in a coffee grinder to make a powder and sprinkle that on top of the soil for calcium loving hoyas.

Thanks. That's a good idea Jimmy.

I guess I'll just stick to the grit I've been using. It's poultry grit (crushed granite).