I have had success in fastening mounting screws into carved limestone to attach it to a separate stone base, by using silicone caulking compound. The type sold for sealing bath fixtures is available in transluscent (cloudy gray) or with an opaque white pigment. This type may be sold in squeeze tubes, and is convenient to work with, as small amounts can be used as needed, and the tube is easily resealed.
Another sort is intended for filling and sealing seams or joints of building exteriors, and is often sold as a silicone and acrylic latex blend. Generally, the greater the proportion of silicone, the higher the price and more lasting the product is claimed to be. This type can also work quite well, but may only be available in 'cartridge' containers for use in 'caulking guns.' Both materials are quite adhesive, and somewhat elastic, an asset where differential thermal expansion could perhaps cause problems, as in joining metal to stone. If the parts are large enough to accomodate it, blind holes can be drilled into the mating broken sufaces, (you'll need to 'spot' the holes so that they meet up, but a small misalignment can be tolerated.) You can then use fairly heavy guage single strand copper wire folded back and forth on itself as reinforcement inserted and imbedded in the caulking, to bridge the matching holes. The holes can be drilled with a carbide-tipped bit used with a hand held electric drill, or an old fashioned star drill will also serve. These materials are likely to be found in shops that sell home painting and maintenance supplies. The adhesive sealants are sold under a number of trade names. In the U.S.A., two common brands are 'General Electric' and 'DAP.' I'm sure that Tony Moss or others on the list will offer other, perhaps better, suggestions. Good luck, W.S. Maddux
