Complex sentence: This method produced a dangerously high degree of long-term residual radioactivity and has been discontinued, although radium-treated green diamonds are still found in markets occasionally and can be detected by a Geiger counter.
Split into simpler sentences: This method produced a dangerously high degree of long-term residual radioactivity, hence has been discontinued. Radium-treated green diamonds are still found in markets occasionally and can be detected by a Geiger counter.