Erm... no they're not. Some are made of Limestone, which is basically Calcium Carbonate, there's lots of granite ones which are (roughly speaking) a mixture of quartz, feldspar, mica etc in varting quantities.
I'm sure someone can furnish me with an example of stone circles made of basic rocks such as basalt or diorite which, although they contain silcate minerals, do not contain quartz as such.
In a rock such as sandstone (which many stone circles are also made of)the quartz grains are complety randomly arranged so any resonant properties would cancel each other out.