The colour of a diamond is determined by a colour scale. The colour white has a gradation of white tones. The Top Wesselton (TW) grade is awarded to especially white diamonds.
The only colour category that is even purer and clearer than Wesselton is the River tone. River-quality diamonds were found mainly in rivers, which led to the introduction of the name River.
On the scale of colour gradation, Top Wesselton is immediately after River, which is called "ultra-fine white" and thus ranks first on the colour scale. In general, however, the term Top Wesselton is only rarely used and mostly regionally, for example in Scandinavia.
By now, the internationally recognized GIA designation is used to describe the colour of a diamond. GIA stands for "Gemological Institute of America". A diamond with the colour Top Wesselton corresponds to the GIA diamond designation of category F (fine white +) or G (fine white).
The term Wesselton is an alternative name for GIA diamonds in the colour H (white). The so called "old terms" refer to the colour designation, which are only rarely used today. They subdivide the colour of a diamond into River (in GIA language D,E), which denotes the finest white and occurs very rarely and represents the most expensive diamond colour.
After this most expensive category follows the clay Top Wesselton (GIA calls it F,G), which hardly differs from River with the naked eye and is also extremely transparent and completely colourless. This category is followed by the colour category Wesselton (GIA language H), which is also colourless for laymen, but has a tiny colour content for professionals. These diamonds are still very rare and have a very attractive price/performance ratio.
After Wesselton the next colour gradation is the Top Crystal (I) tone and Crystal (J,K). Laymen will still classify this tone as white. Professionals however will see a slight discoloration with the naked eye. Diamonds of this colour gradation are most commonly used in the jewellery industry.
The Top Cape (L) and Cape (M,N) colour gradations have a visible yellow tint. That means that the colours of the diamond can still be seen even after colouring. The last colour gradation from pure white is Light Yellow (O-R) and Yellow (S-Z), which are noticeably yellow and are not suitable for fine jewellery.
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