Alum block

I bought some of those from Holland and Barrett on some "Buy one get another one for a penny" deal.
For use on the face I prefer to take some sandpaper to the rounded top until it's flat. Still works for its intended purpose.
If you do buy them from Holland & Barrett as I did, don't be afraid to give the pack a little gentle shake to check if the contents are broken - I went through nearly a whole shelf full before I got a solid one! (No, my 'shaking' did not break them LOL)