Under the action of external force, the fracture of gem minerals that do not follow a certain crystallization direction is called fracture. Fracture is different from cleavage plane, which is generally an uneven surface.

According to the fracture shape, the fracture can be divided into the following categories: conchoidal fracture, uneven fracture, split wood fracture, stepped fracture and so on.

Polychromaticity

In an optically anisotropic colored gems crystal, the polarization absorption and selective absorption of light in different directions are different and polychromatic due to the difference of particle arrangement in all directions of the crystal. The anisotropic colored gems can be bichromatic or trichromatic.

luminesce

Some jewelry and jade will emit visible light under the radiation of external energy (such as heat, violet light, ultraviolet light, X-ray, cathode ray, etc.). ), is luminous.

Fluorescence: When exposed to sunlight, fluorescent lamps or ultraviolet rays from other light sources, some diamonds and other precious stones may temporarily emit visible light, which is called fluorescence.

Phosphorescence: slow luminescence caused by luminescence. The gem with strong phosphorescence is commonly known as the "night pearl".

Gloss: The intensity and characteristics of reflected light on the surface of a gem (smooth crystal surface, cleavage surface or polished surface) are called luster, which largely depends on the refractive index of the gem. The greater the refractive index, the stronger the luster; It also depends on the polishing degree of the gem and the structure of the aggregate. The better the polishing, the stronger the luster.

Gem luster can be divided into six categories:

Diamond luster

The luster of diamond, which includes diamond, rutile, cubic oxide dislocation, dislocation stone and garnet.

vitreous luster

Most gemstones, such as rubies, sapphires, emeralds and tourmalines, belong to this type.

resinous luster

Glossy gemstones with low refractive index and soft texture, some yellow, brown or brown minerals and their massive aggregates, scatter some light due to uneven reflection surface, showing rosin luster. Such as amber, has a typical resin luster, but the polished surface can be close to the glass luster.

pearly luster

A lustrous type resembling a pearl, such as a pearl.

Waxy luster

Gloss is similar to wax, such as turquoise.

Silk luster

The luster type of gems with parallel fibrous inclusions, such as eagle eye stone, is a common gem.

divide

When a crystal is hit by an external force, it can break along the crystal plane or a special direction, that is, a certain direction in the crystal lattice. This phenomenon is called cleavage. Cleavage is determined by the crystal structure of the gem itself. A smooth fracture parallel to cleavage is called cleavage plane.

stick to

The ability of an object to resist wear, tension, compression, etc. It can also be called anti-secession ability. Whether a gem is fragile depends on its toughness, and has nothing to do with hardness. The so-called high toughness means that objects are not easy to break.

Toughness is divided into five grades, from excellent to poor: excellent is very good, excellent is very good, good, average and poor.

Comparison of gemstone toughness (common): black diamond > nephrite > jadeite > corundum > diamond > crystal > aquamarine > olivine > beryl > topaz > moonstone > gold emerald > fluorite.

transparency

Refers to the degree to which an object allows visible light to pass. Can be divided into transparent, translucent, sub-transparent, semi-sub-transparent, opaque and so on.

stability

Refers to the ability of materials to resist physical or chemical changes caused by light, heat or chemical reactions.

Refractive index and birefringence

Refraction refers to the change of propagation direction of light when it enters another medium with different optical density from one medium (unless the incident light forms a 90-degree angle with the interface). The ratio of sine of incident angle to sine of refraction angle is a constant, and this value is called "refractive index".

Isotropic gemstones are birefringent because they are optically isotropic. Anisotropic gemstones have two kinds of refractive indexes, which are called birefringence. The maximum numerical difference between two refractive indices is called birefringence.

The birefringence of anisotropic gemstones is expressed by the difference between the maximum refractive index and the minimum refractive index. For example, a crystal has two refractive indexes: the maximum refractive index 1.553, the minimum refractive index 1.544, and the birefringence of 0.009.

absorption spectrum

Pure white light is a continuous spectrum from red to purple, but when white light passes through colored gemstones, a certain color or wavelength can be absorbed by the gemstones, resulting in one or several discontinuous points in the spectrum of white light, which appear in the form of dark lines or dark bands. Many gems show characteristic patterns of absorption bands or lines in the visible spectrum, and their complete patterns are called "absorption spectra".