I made these 2 necklaces with rocks I picked up at the antique fair last weekend. I dont know the correct name of the little metal backing pieces are, but you will need some of those along with some leather or a chain (I used a chain from another necklace) and E-6000 glue.
take your metal backing and your E-6000 glue and glue it to the back of the rocks. Be sure you place them so you don't see any metal when they are flipped over. The circle part should lay flush with the rock, not sticking out the top. Let them sit for a few hours, or even over night to be sure they are secure. PS. if you don't own E-6000 glue I suggest you go out and buy some because it is perfect for jewelry...or anything for that matter!
Add your chain, or leather, or anything you want to use. See, super simple right? Something that could easily cost you over $20 in stores only cost about $5 to make. SCORE :)