I framed a pair of rock slabs recently, using a technique suggested by a forum member. I've done this before, and it has always come out well.
I used Neilsen Metal Sectional Mouldings for the frame. I got the pieces custom cut by a frame maker, and just had to screw them together and add the wire.
This technique works for slabs are fairly square or rectangular. There is overhang on the frames with will let you fudge a bit unevenness, but there is a limit. Neither of these slabs were perfectly rectangular and they worked fine.
I used the N117 size frame, because it gives decent depth for the thickness of the rock slab and is also has a slim edge for these small slabs. I have use the N95 frame also; it is good if you want a thicker frame, such as for a large slab.
Total price including shipping and hanging hardware for both was $26.61.
Send me a PM if you want to know the online frame shop I used.