Basic Concept of Crystal

Crystal is a single crystal of a naturally clear quartz mineral.

The word "crystal" is derived from the Greek word "KrystaIIos", meaning "white ice".

In ancient China, crystals were called water jade, water turquoise, pale jade, jade Ying, quartz, water essence, crystal, millennium ice, bodhisattva stone, etc.

The English name for crystal is Rock CrysaI.

The mineral name for crystal is -quartz, i.e. low temperature quartz (stable temperature below 573 °C at atmospheric pressure).

The jewelry industry calls -quartz crystal and considers crystal to be the gemstone with the richest connotation and the most versatile.

Classification of crystals

Crystals are classified, by color, into non-colored crystals and colored crystals.

Non-colored crystals include: colorless, milky white, smoky, teal and black; colored crystals include: purple, yellow, purplish yellow, red, pink, green and blue.

By inclusions, crystals can be divided into single mineral inclusions and multiple mineral inclusions crystals.

According to the inclusions form can be divided into
1. Hair crystal (rutile hair crystal, actinolite hair crystal, hornblende hair crystal, asbestos hair crystal, etc.)
2. Water bile crystal (two-phase water bile crystal, three-phase water bile crystal, etc.)
3. Scenic crystals (pictorial crystals of people, animals, plants, mountains, rivers, etc.)

According to inclusions minerals can be divided into
1. Rutile inclusions
2. Tourmaline inclusions
3. Chlorite inclusions
4. Pyrite inclusions, etc.

According to the special optical effect, crystals can be divided into cat's eye crystals, star crystals and rainbow crystals, etc. According to the industrial use, crystals can be divided into piezoelectric crystals, optical crystals, craft crystals and smelting crystals.

Crystals can be divided into single crystal ornamental stones and crystal cluster (aggregate) ornamental stones according to their ornamental properties. It can be further divided into modeling stones, pattern stones and text stones.

 

Physical and chemical properties of crystals

Crystal is an oxide gemstone. Its chemical composition is mainly silicon dioxide, with the chemical formula SiO2, which means that the oxygen atom is twice as large as the silicon atom.

Crystals belong to the intermediate crystal group, the tripartite crystal system. The 32 symmetry types present in crystals are grouped into three families: low, intermediate, and high, based on the presence or absence of a high sub-axis and the presence of one or more high sub-axes. Within each family, the crystals are further divided into seven crystal families according to their symmetry characteristics: equiaxial, tripartite, tetragonal, hexagonal, monoclinic, triclinic and complex. Crystals belong to the intermediate crystal family, the tripartite family, because they have a cubic axis.

Optical properties of crystals

Crystals are uniaxially crystalline and ortho-optical. Crystals are called uniaxial crystals because they have a single optical axis. When light waves are incident perpendicular to the direction of the optical axis of the crystal, the light is birefringent, breaking into two beams of light whose vibrations are perpendicular to each other, and at this time, one group of light is parallel to the optical axis and has a very high refractive index, while the other group of light is perpendicular to the optical axis and has a refractive index equal to that of the normal light. When the value of extraordinary refractive index is greater than the value of normal refractive index, it is called one axis crystal positive light; when the value of extraordinary refractive index is less than the value of normal refractive index, it is called one axis crystal negative light. The non-constant refractive index value of crystal (1.553) is greater than the constant refractive index value (1.544), so the optical properties of crystal are called positive optical properties of uniaxial crystal.

 

 

Birefringence of crystals

The difference of 0.009 between the non-constant refractive index value of 1.553 and the constant refractive index value of 1.544 is the birefringence value of crystal.

Color of Crystal

The color of crystal is other colors. Other colors are the colors caused by trace elements in the chemical composition, or by mechanical admixtures. Crystals of pure silica are colorless, purple with traces of trivalent iron and bivalent manganese, yellow with traces of bivalent iron, green with traces of iron, pink with traces of bivalent manganese and tetravalent titanium, and smoky with traces of trivalent aluminum.

 

Luster of crystal

The luster of crystals is glassy. Luster is the ability of the mineral surface to reflect light. Depending on the intensity of the luster, it can be classified as metallic, semi-metallic, adamantine or vitreous.

 

Transparency of Crystals

The transparency of crystals is transparent to translucent. Transparency is the degree to which a gemstone mineral allows visible light to pass through it. The transparency of gem minerals is usually classified into five classes: transparent, subtransparent, translucent, subtranslucent and opaque.

 

 

The special optical effects of crystals

Special optical effects of crystals: refraction, reflection, interference and diffraction of light can cause cat's eye effect, star effect and rainbow effect in some crystals, which are called special optical effects of crystals.

1. Cat's eye effect: Some curved crystal surface refraction and refraction of visible light caused by a shiny band of light, like a cat's eye, known as the crystal cat's eye effect. As the light source or the crystal moves, the band of light also moves in parallel. Crystals that produce the cat's eye effect must have a dense set of directionally arranged inclusions or structures. In addition, the bottom plane of the crystal should be parallel to the plane of the inclusions.

2. Starburst effect: Some curved crystals show a set of radially shining bright lines on the surface of visible light refraction and refraction, like twinkling stars in the night sky, which is called the crystal starburst effect. Crystals that can produce the starburst effect must contain two or more groups of oriented inclusions or structures, and the bottom surface of the crystal must be parallel to the plane where these inclusions or structures are located. Crystal starlight effects can be classified as three-shot starlight, four-shot starlight, six-shot starlight, etc.

3. Rainbow effect: gemology calls this optical effect "halo color". It is the color produced by the interference and diffraction of visible light under the natural light of some crystal crystals with fissures caused by geological effects, called the rainbow effect of crystal. In fact, all stones with cleavage can produce the iridescent effect.

The special optical effects of crystals can play a role in enhancing the crystal's strengths and avoiding its weaknesses. If a crystal is large and has large internal cleavage, which is obviously a major flaw, but the cleavage can produce the iridescence effect, and the iridescence effect is more pronounced, it becomes a favorable condition to increase the ornamental value of the crystal, and it also plays a positive role in increasing the economic value of the crystal.

 

 

Mechanical properties of crystal

1. The density of crystal is 2.65 g/cm3, which is the mass per unit volume of the stone. The unit of density is g/cm3, which depends on the atomic weight of the elements that make up the crystal, the atomic radius and the crystal structure.

2. The hardness of a crystal is 7 degrees on the Mohs scale. Hardness is the resistance of a gemstone to being pressed, scratched or ground. The hardness of a gemstone is related to its chemical composition, chemical bonding and crystal structure. The smaller the distance between the atoms in the crystal, the greater the bonding force and the higher the hardness; generally speaking, the greater the atomic value, the higher the hardness of the stone. The higher the atomic value, the higher the hardness. The high hardness gives the gemstone its durability, which is one of its most important properties.

3. Crystal has a toughness value of 7.5, which refers to its resistance to tearing and breaking and corresponds approximately to its brittleness. Toughness is also known as strike hardness. Toughness is related to the structural configuration of gem minerals. Jadeite, Hetian jade and agate are not very hard (6.5-7), but their toughness value (8) is higher than that of crystal because they have a fibrous or cryptocrystalline structure.

4. Crystals are extremely non-disintegrating stones. Dissociation is a trait in which the crystal splits into smoother planes along a specific crystallographic direction under the action of an external force. The plane of cleavage is called the plane of deconstruction. It is most likely to occur in the direction of the weakest atomic or ionic linkage within the crystal.

5. The fracture of a crystal is shell-shaped. A fracture is a random, irregular fracture of a gemstone without a certain direction, caused by an external force. Common fractures are shell-like, jagged and earth-like.

 

 

Electrical and thermal properties of crystals

1. Crystals have a good piezoelectric effect. Single crystals of higher clarity produce an electric charge when subjected to pressure; conversely, when subjected to voltage, they produce a high frequency vibration. This property of crystals that allows pressure and charge to be converted into each other is called the piezoelectricity of crystals. The amount of charge generated varies depending on the pressure, and the frequency of vibration varies depending on the voltage. Both natural and synthetic crystals have good piezoelectric properties and are therefore widely used in radio and remote control resonance devices.

2. Crystals have good thermal conductivity. Thermal conductivity is the ability of a gemstone to conduct heat. The thermal conductivity of different stones varies greatly, so it can be used as one of the identification characteristics of a gemstone. Thermal conductivity is expressed in thermal conductivity (K), in W/(m.K). Gemmology generally expresses the relative thermal conductivity of a gemstone in terms of its relative thermal conductivity. If the thermal conductivity of spinel is taken as the base 1 (281), the relative thermal conductivity of crystal is 0.5-0.94 (140-264). The relative thermal conductivity of glass is 0.08. Therefore, crystal is cooler than glass.

By sharing the above basic crystal knowledge, can you have a new understanding of crystal now?