These are sinter stones from a volcanic area in Germany. They are not mainly of calcium, but of harder and heavier materials. All these are outstanding. I purchased these last week to always have the right slab for a tree or group. I might sell some. They are not cheap! One has to compare them to the crescent pots. These are the real thing and one can tell. Why shold they be cheaper than the copies? They are hardy.
Here is one with spruce planted on it.
And then here is my loot of last week. Most of them are shown twice, turned upside down. They are full of character, color, texture. I am exited.