be careful with the boiling...any internal air pockets could cause an explosion...
the fizz will only tell you if has or is made of carbonate...which if fine if you want to raise or maintain a higher pH.
I've personally never had an issue or know anyone personally who has had any problems with rocks they've found. The only rock I've wanted in a tank that I've held off on is a piece of calcite.
Granite, chert, slate, miscellaneous silicates, etc...pretty much all hard rocks are fair game as long as they are not salt.
I have a 75 gallon cichlid tank that acts as a display tank for a lot of the neat looking rocks I find. The entire substrate and all decorations are rocks I've picked up. Arrow heads look good mixed in.
edit: other found items like deer antlers look good with these things as well...they grow some algae and such and gradually change over time with some flaking and produce some cool looking patterns when mixed with some larger rocks and plants look really good imho