Since you are purchasing for faceting rather then display, make sure that the transparency and color are good, and make sure you are getting it from a reputable dealer to ensure it isn't a fake or synthetic. Also get the origination source - i.e. Colombia, Brazil, Zambia, etc. Make sure to ask if it was oiled, or if treated in a way outside of the industry standard. And check for flaws -- heavy flawing will obviously decrease the value, though minor flaws can be desired in Emeralds as they show authenticity.