Purple of cassius is only a form of colloidal gold. It is purple or red in colour and after the name of Cassius who discovered it.
It is prepared by mixing very dilute solution of gold chloride with stannous chloride solution.
2AuCl3 + 3 SnCl2 → 2Au↓ + 3SnCl4
The gold thus precipitated is adsorbed by stannic hydroxide formed by the hydrolysis of SnCl4.
SnCl4 + 4H2O→ Sn(OH)4 + 4HCl
It is used in making ruby-red glass and high class pottery.