The term Wesselton refers to an outmoded point on the diamond color grading scale and is equivalent to a color grade of H on the new standard jewelers' color scale. Diamonds are graded based on how close to a "white" or "pure" color they are in their cut state, and Wesselton and Top Wesselton diamonds are very near to total purity.
A diamond graded white by the old standards of the International Diamond council would receive a designation of Wesselton. The only grades above this were fine white River and Top Wesselton diamonds and finest white River and Jager diamonds, very rare and extremely valuable.
There are three scales of color grading, the alphanumeric scale being the current industry standard in the Western world as of 2015. The other two are the English Denomination and the Old Denomination, which take a much more fanciful approach with their prosaic descriptions of diamond quality and the colors contained in a stone.
The English Denomination runs from tinted color to finest white while the Old Denomination runs from Jager to Light Yellow. The Color grade, also known as the alphanumeric scale, simply runs from D as the highest-graded stone to P-Z at the end of the scale with high-color and high-tint stones.