It is well known in the trade that most emeralds on the market today are oiled. This article focuses on Colombian emeralds and gives a detailed explanation of the oiling process as observed by the author. Sophisticated laboratory procedures are compared with common "back porch" methods. The methods used often vary from parcel to parcel, and the success of the treatment depends in part on the experience of the treater, although the process itself appears to be relatively permanent. Observations are also presented on methods of detecting oiling as well as on the use of colored oils.