Opal is a kind of gold emerald in mineralogy and belongs to spinel mineral. Gold emerald is an alumina containing beryllium, and its chemical molecular formula is BeAl2O4. It belongs to orthorhombic system. Crystal morphology is often short columnar or plate-shaped. Opal has many colors, such as honey yellow, brownish yellow, wine yellow, brownish yellow, yellowish green, yellowish brown, grayish green and so on, among which honey yellow is the most precious. Transparent to translucent. The glass is greasy and shiny. The refractive index is 1.746- 1.755, and the birefringence is 0.008-0.0 10. Dichroism is obvious, the dispersion is 0.0 15, and it is not isotropic. The hardness is 8.5, and the density is 3.71-3.75g/cm3. Shell fracture. The difference between cat's eyes like opal and artificial cat's eyes. Tourmaline, beryl, apatite, Yingshi, kyanite, etc., which can produce cat's eye effect in nature, are not as precious as gold-green cat's eyes. Tourmaline cat's eye: hardness 7-7.5, density 3.06g/cm3, refractive index 1.624- 1.644. Quartz cat's eye: low hardness, about 6.5, low density, 2.78 g/cm3, low refractive index, 1.44. Artificial cat's eye: opal made of artificial glass fiber, with brown, blue and red colors. The artificial opal has 2-3 bright bands at the arc top, while the natural opal has only one bright band. Observing with a magnifying glass, we can see that the artificial opal has a hexagonal honeycomb structure, with a low hardness of about 5, a low density of 2.46 g/cm3 and a refractive index of 1.44.