These cufflinks are made from antique watch movements. The movements are soldered to silver plated posts, and are very strong.
These watch movements have tiny silver or gold gear and cogs, and tiny pieces of ruby (note the red specks). Ruby was used as bearings since steel bearings would wear out much quicker.
Your pair may not be identical to the set in the picture, the shape of the watch may vary, but your pair will be as close to an identical match as possible.