Sharpening Stones?

[Robert]

Yes, some stones do work better then others. Your average pebble is more often then not quartzite. It's pretty hard rock and takes a lot of wear. It has hardly any grain. This also makes that it doesn't "cut" iron very well. It's fine for removing burrs and picking up an already existing edge. A sandstone is just that, a stone made of bonded sand. This has a lot of grain and can/will scratch and score your iron, crumbling somewhat as it does. There are a lot of grades in texture in sandstone (just like in sandpaper), so it can be selected to find a fine grained piece. The chances of finding sandstone as a pebble is not very high, as it will not survive fluvial transport. It can make an excellent sharping stone. A granite or basalt on the other hand has a very hard, more crystalline rock, with more "cut" then a pebble. The Romans were also very into tuffstone, a volcanic material. This has great sharpening characteristics, fine grained, not to hard, nice cutting action.
Then there is the mudstone, this has a really fine grain, and is great for getting a razor's edge (wet the stone!), but isn't very strong, so be gently on the pressure.
For best results, best to combine different types of grain/hardness, working from coarse/softer to really fine/hard.