Spangold
Some gold copper-aluminium alloys form a fine surface texture at heat treatment, yielding an interesting spangling effect. At cooling, they undergo a quasi-martensitic transformation from body-centered cubic to body-centered tetragonal phase; the transformation does not depend on the cooling rate. A polished object is heated in hot oil to 150 - 200°C for 10 minutes then cooled below 20°C, forming a sparkly surface covered with tiny facets.
The alloy of 76% gold, 19% copper, and 5% aluminium yields a yellow color, the alloy of 76% gold, 18% copper and 6% aluminium is pink.
Read more about this topic: Red Gold