I bought a big (150ml) tube of Williamsburg Egyptian Violet yesterday, and thought I should write something about it here.
Many oil paint manufacturers call this colour Dioxazine Violet, and the chemical composition of Willaimsburg's is "Carbazole Dioxazine": PV 2 (Pigment Violet 23), listed on the back of the tube.
I haven't tried Dioxazine Violet by any other manufacturer, so I don't know if this is true of any other Dioxazines, but it is certainly the case with the Willaimsburg; it has a golden sheen! Areas painted with this colour will show this rich gold undertone colour wherever there are highlights.
Anyway, it's a beautiful colour, and if you don't have a deep Violet in your collection, you ought to give Williamsburg's Egyptian Violet a try!