Casein identification guide: casein is a thermoset semi-synthetic polymer made from milk, hardened in formaldehyde
8. Takes a high polish, but the back may be unpolished or less polished, helping to differentiate from Bakelite, which is tumble-polished on all sides. The back of this button shows the dull finish sometimes seen on casein. Great imitation of marble.
source: Button Materials A-Z: Identification Guide
by Jocelyn Howells