The screen door in front of the Crystal Gems' house sometimes opens in and sometimes out.
This could be incorrectly regarded as a goof since the Gems don't understand Earth technology. Maybe they installed the door that way.
A pearl is a gem, but it is not crystalline. This fact was acknowledged in the show - Peridot said at one point that pearls are not real gems, and are thus considered to be inferior to all other types of gems.