Are you using a wetsaw? I'm thinking that you could make an adjustable box like jig that would rest on the tray and cradle the rock, then use a chunk of wood as a press to hold it down while you slide the saw. If using a dry blade on a hand held saw, it would be difficult to stabilize the rocks without the help of a partner

Are these smooth tumbled rocks (pebbles) or beach glass?

They are egg shaped natural stones. Mainly quartzite, hard shale and felspar.

Here is a link that might be useful: Indirect Method

Thanks for the advice nicethyme, my mind was running along the lines of the box too - but not so sure about just pressing down with piece of wood. I've tried that kind of thing already , but with smallish oval pebbles about the size of a goose eg and smaller, it's damn dangerous, as these things slip around like mad. Clamping is difficult as you seem only seem to to pick up two points at a time and then it pivots and slips.
I thought of epoxying one end (sacrificial) to a square wooden block and then clamping the block and pebble together onto the cutting bench, so that it can't rotate or pivot, but I don't know if the epoxy glue would be strong enough. If it came loose, the saw would throw it out like a bullet.

What do you think?

Cape Town

I had some success with the epoxy - but it is rather messy and time consuming, so I'm still looking for a better method - any further suggestions.

Cape Town

First wrap the pebble completely with adhesive tape, to prevent shards flying, then use the same tape to splice a short piece of wood to either side of the pebble to make a sandwich. Then hold the entire sandwich firmly in a vice or clamp. You can then safely make a cut with diamond disk at least a centimetre or more deep. Then place your pebble sandwich onto a solid concrete base, propped between two bricks to hold the sandwich with the new cut uppermost. Put the cutting edge of a bolster (Bricklayers flat chisel) into to the cut you made and making sure that the chisel is held vertical, give it a very sharp tap. Be sure to wear safety glasses.

WOW! Thanks for the info Toby!

(either before or after the cuts)!

I have not tried this yet, but think this would work well, provided you are in possession of a wet stone cutter. Take a flat square plastic container of appropriate depth, ( at least 40mm ). Rub some form of oil around the container or line with clingfilm. Take a batch of your beach or river stones and wrap each one in clingfilm. Place them in the container, leaving them at least 20mm apart, (or push them in afterwards) depending on size. Mix a small batch of rough concrete in a bucket and gently pour over and tamp down until the top of the stones are only just visible. Leave to dry for a day or two. You should end up with a small square slab, manipulate the cotainer a bit and it should come out easily. You should be able to cut the stones precisely and then break the slab up afterwards.