Ken, You'll not find rocks like that on this coast. The volcanic rocks on this side of the country will be granite, gneiss, or similar. Basalt, obsidian, and tuff are formed by different types of magma eruption/cooling than we had here. We have good stuff, but not flat black or dark grey like basalt.... a pity, really.ryuken168 said:
You may be able to find some nice black rocks (usually a schist-- these are metamorphosed sedimentary rocks), but they will be high in mica (both main varieties) and therefore have a striated, metallic appearance. Nice, but not the same. I find tons of the stuff in a local creek. I still haven't used it much in aquariums.
The vast majority of volcanic rock to be found around here is going to be granite. In my area, there is a special deposit of gneiss known as blue rock which I use and like very much. You might find some of that in a drainage ditch near you.
There is granite to be found which is high in hornblende (black crystals) and biotite (the darker mica) crystals and is much darker than run-of-the-mill granite. It is commonly used as counter top material. I don't know where this particular variety is quarried.
Anywho-- hope this helps...
