Oddly enough it's pretty much what legend says it is, but it's a powder. First stage purification derives the white powder which can be used to create the elixir of life. Continued purification creates the red powder of projection which can turn base metals into gold. Additional purifications amplifies the potency.

However, there was one man who could apparently also create gemstones of the rarest quality using the alchemical process, but there is no information that I've come across that tells of that particular process.

Personally I believe the physical process is a mirror to the internal process of soul purification and enlightenment and St. Germain was using manifestion techniques to create the gemstones (much as Jesus did with the fish).