Vintage Agate Quartz (?) Rock Floating Modernist Sterling buying Silver Brooch Pin Pat No 2066969

Pretty brooch that I am guessing is Agate? Quartz? I just don't know! It is floating and in a modernist backing. Back is marked Sterling with the pat no 2066969 on it. I'm not sure who the maker was.

The rock has some tiny natural flaws in it but feels smooth all around an not chipped. It measures about 2 1/2" long. Back shows some light age and use wear. Please see photos for the best description! buying