Almost all pearls are treated for luster in one way or another. Mostly they are simply polished in a polisher (vibrating drum) using salt or sand or even bamboo. This is done only for a few minutes at a time as not to damage the surface of the pearl. This process naturally brings out the deep luster and exposes any potential desirable orient. This is completed prior to stringing into hanks.
The luster treatments based on coating a pearl with a clear coating is one of the undesirable processes in the pearl industry. The coating will probably appear fine at first, but over time will then end up cracking or peeling as the pearl becomes older. This will in turn leave only the dull luster appearance underneath the coating. This is something to watch out for because dishonest pearl processors and traders have fooled consumers in the past and will continue to attempt to do so.
To detect luster treatments, one can look at a pearl through a magnification of at least 50X. A natural untreated nacre surface is somewhat scaly, while a treated surface will appear smooth and man made.