Selling gems and jewelry always fetches 80% of the object’s value unless the DM states otherwise.¹⁾ Characters do not lose any XP that has been awarded for these objects by selling them, since they are already in transportable form before being sold. Of course, if the object is sold prior to XP being awarded, such as to split the value among characters, then the XP is awarded based on the resultant coinage, which will be in total 20% less XP than a single character would have gotten had they kept the object.

This means there is a bit of strategy in choosing whether to sell or keep these items. On one hand, more XP and GP. On the other hand, it has to all go to a single character.

When looted, spellbooks have a treasure value of 100/200 (xp/gp) per spell level contained therein.²⁾

See Magic Item for how to handle all these cases.