Purple gold
Gold-aluminium phase diagram: the top axis is percentage gold by mass, the bottom axis is percentage gold by amount of substance[clarification needed], and the left axis is temperature in degrees Celsius
Purple gold (also called amethyst gold and violet gold) is an alloy of gold and aluminium rich in gold-aluminium intermetallic (AuAl2). Gold content in AuAl2 is around 79% and can therefore be referred to as 18 karat gold. Purple gold is more brittle than other gold alloys, (a serious fault when it forms in electronics[10]), as it is an intermetallic compound instead of a malleable alloy, and a sharp blow may cause it to shatter.[11] It is therefore usually machined and faceted to be used as a "gem" in conventional jewelry rather than by itself. At a lower content of gold, the material is composed of the intermetallic and an aluminium-rich solid solution phase. At a higher content of gold, the gold-richer intermetallic AuAl forms; the purple color is preserved to about 15% of aluminium. At 88% of gold the material is composed of AuAl and changes color. The actual composition of AuAl2 is closer to Al11Au6 as the sublattice is incompletely occupied.[2]
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