I actually find that natural stonze are too light for any meaningful purpose.
If its the camo angle you are after have a look at the `Pebbles' from Anchor Tackle. http://home.btconnect.com/anchortackle/ancpebbles.htm
They are actually proper leads but with totally irregular shapes and fantastic camouflage.
They can be obtained from :- www.hookedangling.comjust click on the Order Anchor Tackle retail link.
Dragon carp in Hucknall Nottm have a fair stock of "stonze" although as the "Coach" said they are a little light and not,by their very nature, very aerodynamic. I do however have a fair few which I use along with some home made jobs that require nothing more than araldite and aquarium grit which is glued to standard ledger weights. Dragon do a mail order service by the way.
There are also various powders available to alter the colour of leads into a variety of different colours including gravel they may be of interest to you.