Cat's eye, namely "cat's eye", "cat's eye" and "cat essence". Opal, also known as oriental cat's eye, is a rare and precious variety in jewelry. Opal is named because it is as flexible and bright as a cat's eye and can change with the intensity of light. This optical effect is called "cat's eye effect". There are many gems with cat's eye effect. Gemmologists call the chrysoberyl with cat's eye effect opal. Generally speaking, opal refers to emerald-green cat-eye gemstone, while other gemstones with cat-eye effect must be preceded by the word "cat's eye", such as aquamarine cat's eye and tourmaline cat's eye.

The chemical formula is BeAl2O4.

Crystal system: it belongs to orthogonal crystal system.

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,

The density is 3.71-3.75g/cm3.

Origin: Opal is also found in Bula and Goyle in Trana, and in the southwest of Sri Lanka, Brazil and Russian countries.

Opals are mainly produced in hydrothermal deposits and pegmatite veins. Opal is a kind of gold emerald in mineralogy and belongs to spinel mineral. Gold emerald is a kind of beryllium-containing alumina, and its crystal shape is often short column or plate. 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. Shell fracture.

The difference between cat's eyes like opal and artificial cat's eyes. In nature, tourmaline, beryl, apatite and kyanite can produce cat's eye effect, but they are not as precious as gold-green cat's eye. 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.

Evaluation and purchase of opal. The evaluation of opal is based on the color, the position of eyeliner, the shape and weight of the gem. High-quality cat-eye gemstone, the cat-eye line should be thin and narrow, and the boundary should be clear; Eyes should be flexible to open and close, showing vivid light; The color of the cat's eye should be in sharp contrast with the background; And the cat's eye line should be located in the center of the cambered surface. When purchasing, we should distinguish opal from other precious cat eyes and artificial cat eyes according to the identification characteristics, and purchase genuine opal.

In Southeast Asia, opal is often regarded as a symbol of good luck, and it is believed that it will protect the owner's health and longevity and stay away from poverty. Opals are often called "noble gems". It belongs to one of the five precious and high-grade gems in the world together with the variable stone. Hope, a British gem collector, collects a famous opal, which is carved into the shape of an altar with a torch at the top. The whole gem is spherical with a diameter of about 1- 1.5 inches.