The cause of discoloration of metamorphic rocks can be explained by the absorption spectrum of metamorphic rocks. Some emeralds selectively absorb light, orange, yellow and green light, and basically do not absorb red light and green light. In this way, the gem will appear green when the light intensity of green wavelength is in the sun during the day, and red when the light intensity of red wavelength is in the night.

The main factors affecting the price of color-changing stone are its color-changing effect, size, transparency and the number of defects. Generally speaking, the more obvious the discoloration effect, such as emerald green during the day and bright red at night, the higher the price. However, under normal circumstances, there are very few gems with such obvious discoloration effect. Generally, they are dark green during the day and reddish brown at night, which is like dark green tourmaline during the day and garnet red at night.

Nowadays, the output of metamorphic rocks is extremely rare, and the main producing areas are Brazil and Sri Lanka. Metamorphic rocks, like cat's eye, belong to chrysoberyl in mineralogy, but they are two different gems because of their different optical characteristics. Metamorphic rocks belong to orthogonal crystal system, and the crystals are mostly short columns and plates. Can change the color (green, red), transparent, translucent to opaque. The refractive index is 1.745- 1.754, which is strongly dichroic and anisotropic. The hardness is 8.5 and the density is 3.73g/cm3. Excellent toughness. Weak red light can appear under both long-wave and short-wave ultraviolet irradiation.

The difference between variable gems (emeralds) and similar gems. At present, there are artificial corundum metamorphic rocks, artificial spinel metamorphic rocks and synthetic metamorphic rocks on the jewelry market for sale as natural metamorphic rocks. The main basis of naked eye identification is polychromatic, inclusion characteristics and particle size. Natural metamorphic rocks are dark green or fluorescent green in the sun, and the particle size is less than 1 cm. It's tricolor. It has gas and liquid inclusions. Artificial corundum metamorphic rocks are only dichromatic. Synthetic spinel is not polychromatic. Artificial corundum metamorphic rocks and artificial spinel metamorphic rocks have gas-liquid inclusions and arc bands. The most reliable identification method is to determine its density.

Evaluation and purchase of metamorphic rocks (emeralds). Metamorphic rocks (emeralds) should be evaluated and selected on the basis of perfect discoloration effect. In most cases, the stone changes from crimson to purplish red under the light; It is yellow-green and blue-green in the sun. The best emeralds and rubies are rare.

The main producing areas of Alexandrite are Sri Lanka and Brazil, others are Myanmar, Tanzania and Zimbabwe, and the mineral deposits in Russia's Urals Mountain have almost dried up. Emerald with beautiful color change is expensive, and high-quality metamorphic rocks over one carat are even more expensive than high-quality rubies, sapphires and emeralds of the same size. But even if you have enough money, it is extremely difficult to get such a gem. It is really difficult to find it.