I tried using a silverfin skeleton as a reagent (as the conjurative sand indicated that is was one)

The first time, it consumed a knuckle to create a lesser ghost. I dropped all knuckles to see if it would do the same; instead it consumed a goblin ear charm, and summoned again a lesser ghost. I take it's bugged as it should have used the silverfin skeleton instead of some random object.

This isn't really as much of a bug as it may appear -- reagents that have the same effect are consumed interchangeably. I am going to revise the system to not do this, but it's not a high priority issue (consuming random objects would be a high priority issue).

Are goblin ear charms reagents? If not, then it seems to be consuming random objects instead of the reagent.

I suspect the goblin ear charm simply (unintentionally) shares a tag, since the way most loot gets made is editing items. So this is sort of a bug, but not a huge one.

Fixed. Reagents are no longer selected by tag.

On a sidenote, this means that reagent choice will no longer persist over resets -- but that wasn't a major feature (and could potentially be confusing).