They put the radium paint brush tip in their mouth to fine it to a point easier. The radium salt went through their oral issues and was incorporated into their jaw as radium acts like calcium, which got blasted apart from alpha radiation.

With an intact crystal, you mostly get some beta radiation beaming out of the watch face and some light gamma radiation emanating from the watch like a bubble.

I bought an old aircraft instrument gauge today, now the radium in that makes the Geiger counter tick a bit more than a wrist watch!