Oiling is a process of applying mineral oil to a stone, and is used to mask inclusions or enhance the color of a stone. Emeralds, although very valuable are typically infused with inclusions. Emeralds are typically oiled to improve quality.
This technique is commonly used on emeralds. The purpose of this technique is for the oil to fill the fine cracks that weaken the green color. The oil fills the cracks making them "disappear" and thereby improving the color.
Oiling is used to seal a stone such as opal from water loss. It is used to fill fissures on the surface of emeralds, improving their clarity.