Precipitated gold crystals. Coloured scanning electron micrograph (SEM) of gold particles that crystallised over several years from a colloidal gold solution. A colloid is a substance microscopically dispersed throughout another substance. Gold colloids are used for red colouration of glass, aggregated colloidal gold gild porcelain and glasses. The optical and electronic properties of colloidal gold promise great advances in microelectronics and microsensors. Magnification: x26,700, when printed 10 centimetres wide.