TW VS or WSI
Top Wesselton (TW)
The Top Wesselton (TW) grade is awarded to especially white diamonds and ranks in the top on the International GIA scale (F-G), the only colour category above is river. Those diamonds are mainly found in rivers instead of mines.
VS - Almost flawless. Under a microscope grading experts notice even the slightest inclusion, but to the naked eye a VS diamond is extremely clear and appears flawless. It is a fine choice.
The Wesselton (W) grade is next in line and lands an H (near colourless).
SI - Slightly included. Minor small traces exist within SI diamonds but are generally only visible with magnification to a trained eye. A valuable and popular choice.