Size - Larger pearls command higher prices and exponentially so as they approach the top of their potential size range.

Shape - Round pearls are the most valuable, although not always the most desirable for every application. Perfectly round pearls can be significantly more expensive

Color - Beautiful color is related to price but is also a very subjective matter.

Luster - The shinny and reflective quality of the pearl's surface.

Iridescence - also called orient, is the ability of the pearl to break light into its fundamental colors like a prism.  This characteristic is one of the pearls most mesmerizing qualities.

Surface Quality - This refers to the number of blemishes.  The fewer, the better, obviously, but even great pearls can have a few minor "birthmarks".

Make - Although not a quality characteristic of the pearl itself, "make" refers to quality with which the jewelry is assembled including the care taken in matching the pearls used.  The pearls in the piece should complement one another so that the overall affect is pleasing.  Current fashion can often call for the use of mismatched pearls in terms of color and size.