If you can't dig out the stone in the spots where your fixture spike needs to be, then you can try an alternative. Use a 1 1/2 inch masonry drill bit and drill a straight down hole into the rubble... For your stake... use a 1 inch PVC pipe with a 1 x 1/2" female adaptor on top.
You can start with a piece of the pvc longer than you need, bang it in with a rubber hammer or a block of wood... so you don't shatter the pipe. then make a clean cut at the desired height to which you use PVC cement and glue your cap on. Make sure you have a small hole drilled into the side, just below grade ideally, for your fixture wire to exit. You could also make two holes in the pipe to allow for the supply line to enter, make your splice and then exit off to the next fixture.
The fitting I'm talking about will glue on to your 1 inch pvc and the other end of it has a female 1/2'' thread.
Paint the exposed PVC to match the fixture or surroundings.