Diamonds are sometimes coated or painted to improve their color grade. Such coatings can be applied the bottom culet facet, to other pavilion facets, to the girdle or underneath the prongs. Flouride is sometimes used to coat yellow diamonds to make them appear colorless.

Touching the bottom or side of a slightly yellow diamond with a purple marker can make a diamond look whiter. This can easily be detected by washing the diamond in alcohol.

Another technique used called "sputtering" involves coating the bottom of a diamond and then mounting it in a full-back setting where it is harder to detect. This substance will not wash off so is harder to detect. One can improve the color up to seven color grades in this manner. Sometimes chemicals can be "baked" on.

Coating or painting diamonds is not a permanent method of improving the color of diamonds. As such, it is not disclosed to the buyer and is considered a deceptive practice.

You can detect a coated diamond by cleaning the diamond with a solvent or by using a color filter.