Granite is relatively inert rock and I don't think most kinds would contain potentially problematic metals or other issues. But it is very heavy.
Can you find limestone in your area? Apparently Southern Ontario bedrock is mostly Paleozoic marine deposits which must include some limestone. Limestone is often interestingly featured and various plants really like to grow on it. And it's much easier to cut than granite.
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I would LOVE to use the dolomitic limestone as it is all over my farm and, as you note, very beautiful. Lots of mosses and bryophytes grow on it. However, everywhere I read it's highly problematic to use calciferous rock since it makes the pH rise and tropical plants, Bryophytes & mosses require low pH.
I can't speak to this map but, on the ground here I can tell you it's limestone and granite. Interesting fact: my farm is over karst and adjacent to my farm down the hill, all those properties on the west side only have 1" of soil while all the properties to the east of me degrade into gravelly soil. My farm however has a special alluvial deposit giving me 4' of amazing topsoil, thanks to some ancient glacier.
While I discuss pH, am I right in reading dart frogs want a higher pH? So confusing since jungle plants want lower pH. How can it be? There first two builds are practice runs for my dart frog vivarium.
Limestone is heavy too but, I have shale here which is thin. My intention with the heavier rock is to use egg crate siliconed to the back, mixing cork, some rock, driftwood to landscape and provide plenty of interior spaces for the many epiphytes I have. Do you think silicon is a strong enough adhesive to attach egg crate to glass and to rock?
Thank you for helping me wade through this rock issue. The living room is littered with all the build materials and the eco terra stands waiting.