A brown envelope, with a lump in the middle of it that keeps it from sitting perfectly flat. When opened the envelope contains about ten pages of Alchemy, coded in Mustang's own code, detailing his testing efforts and the ingredients, as well as the steps he took to achieve what was in the small box.
Inside the small box, of course, was a raw diamond-- a good three karats, uncut. Once cut, the purity wouldn't be perfect, but there was no need for the test to be perfect. Alphonse was only looking to give Mustang a little help, though he hadn't asked for it. It definitely needed to be tweaked, and fixed, and finalized.. but it was a starting point.