Abstract: Blue opal from Peru is a new variety of gemstone. The comprehensive research on the gemmological features of this kind of blue opal shows that it is composed mainly of opal and minorly of α christobalite and α tridymite. At high temperature (1100℃), the opal, α christobalite and α tridymite in the blue opal are thermally transformed into β tridymite. The water in the blue opal occurs mainly in the form of planar water and structural water, with its Cu²⁺ impure ions tend to combine with H₂O molecules into Cu²⁺(H₂O)₄²⁺ hydrated ions. The presence of 761nm ²Eg→ ²T_1g spin permitted transition absorption band in the octagonal crystal field indicates that the colouring of the blue opal is attributed mainly to Cu²⁺ (H₂O)₄²⁺ hydrated ions.