I got two gemstones which proved to be unsellable, marked as stolen.

These are normal gems, nothing indicated they would be something special or one of a kind types which could mean that they can be recognized as stolen so merchants would not take it. They are pretty ordinary gemstones.

I suppose it is a bug?

What type, where did you get them from, and how?

A fire opal and a topaz. It was part of my share of loot after making the Urdlenites in the mines quest.

Snoteye ... and how?

She is refering to the ones on the floor I beleive.

I noticed this as well, none of those gems are sellable.

This is why I need to know where they're from.

Mystic_warden It was part of my share of loot after making the Urdlenites in the mines quest.

This is done for a reason I think, if those gems become to value able and Urdlenites will be done ONLY for those gems. <.<. Stands to reason I think.

Since the original poster has still not answered my question, I'll take it to mean that Cruzel was right in his assumption, in which case there is no bug.