The normal skeleton mobs also have a chance to drop the earring. Just saw it announced in chat.

Dropped by "Skeleton Witmirth"

Skeleton Archer, Skeleton Wolf, Skeleton Warrior and Skeletons drop them as well.

I've killed thousands of monsters at Hex Sanctuary (easily more than 80,000), so far the chance of this seems to be .1% (one in a thousand), reducing it to around .15% (one in 666) if you have Rank S Knowledge on all mobs and Node lvl 10, that plus Kamasylve Tree buff (20% more drop rate) seems to make it 1 every 500.

If you melt them, you get sharp black crystal shard.
http://bdocodex.com/jp/item/4998/?
Got 1x 1 sharp, gold and ruby.
and second try 2x sharp.