Another stone question, sorry.

[Rorschach]

Car booting this morning I spotted what looked to be a very dirty but hardly touched stone amongst the pile of filthy and well worn out carborundums and India stones. It had the look of a natural stone and under the grime it felt smooth, very fine and very hard so I took a punt. I gave it clean with some degreaser and I think it's some kind of slate but I am likely wrong, I don't have much expertise in this area.

Stone is natural I am certain, it has quarry marks on the edges from cutting and shaping, the colour is very dark grey/blue, I can't see any "grain" to it at all, it seems pretty uniform. It doesn't appear to absorb any water at all and is very hard and smooth as I thought. Tomorrow I will get out some paper and glass and lap it, hopefully helping identify it, but since I am not allowed any workshop time time today is there anywhere I can start to possibly identify it, anything I should look out for?

 

[essexalan]

I have seen suggested (Henk?) using the back of a stainless teaspoon and see what sort of finish you get. No idea how you identify one sort of slate from another unless you buy it from a known supplier cut from a known quarry. Even then it appears the slate changes in the same quarry. Try it before you dress it with some water and plain HCS and see how it behaves and I hope it is a good piece.

 

[ED65]

Stone IDs can be a bit of a guessing game even going from photos, unless there's something unique about a stone there's no way anyone can help you narrow it down from a written description.

 

[Rorschach]

I'll lap it and take some pictures tomorrow, hopefully will be able to narrow things down a bit.

 

[adrspach]

I have seen suggested (Henk?) using the back of a stainless teaspoon and see what sort of finish you get. No idea how you identify one sort of slate from another unless you buy it from a known supplier cut from a known quarry. Even then it appears the slate changes in the same quarry. Try it before you dress it with some water and plain HCS and see how it behaves and I hope it is a good piece.

Henk was using his spoon to recognise the finish as well as if there were any inclusins and hardness. It was not very scientific and he was planning to make a machine/ tester where he would use a bearing ball of known surface polish to slide in predetermined number of times with constant preset presure and speed to make a mark which could be then collected and archived for comparison with known hones.

 

[Trafalgar]

If you've de-greased the stone, and I think you mentioned that you did, I'd simply hone something on it to see what happens. Maybe do the flat back of an already honed plane iron and try to notice the degree of polish before and after. You may determine that the stone offers no advantage to your current kit and you will have saved all that time you'd planned to spend lapping it.