I did a bit more stone carving whilst I had the loan of the side grinder - it has now gone back to its owner. The opening photo is one of my favoured shapes and simple cross over grooves - though the stone is a bit granular and does not cut as smoothly.
Photo of the stash of stones.
And some individual ones. The stone with the white stripe and three grooves is quite large and weighs over a kilo.
I would love to be able to gather some more black stones - but they appear to be a bit rarer.