Measures from .635 - .640 inches or 5/8 on an inch. This marble is in near mint condition. This is a nice flame marble with medium blue swirls and flames on an opaque white base. Lots of action on this marble, look at all the pictures. There are a few small pin pricks and a tiny, shallow nick, about 1/16 of an inch across, not visible from the display side of this marble.