The coloring looks like...

Dude, (sorry I'm not a dude kind of guy but seriously, dude) you found this rock in an antique store, which means your rock is old. Now, obviously, all rocks are old, but yours was mined way before anybody bothered making note of veins from just one mine.

For that reason, my idea for an ID is...
it's a coticule.

Have fun with it!