The General Knowledge to Crystal

Posted by SandyC on April 17, 2009

Crystal is a colorless and transparent quartz crystal minerals. Its main chemical composition is silica, which is the same original mineral as ordinary sand. Crystal is just the perfect crystalline silica; silica gel after dehydration is agate; the solidification of the gel of silica with water becomes opal; when the size of silica grains is smaller than several microns, chalcedony, flint and secondary quartzite come into being.

Perfect crystalline crystal belongs to hexagonal crystal system, showing a hexahedron of a column with a sharp tip or two, and many long columns are linked together, commonly known as crystal family, which has magnificent beauty. Incomplete crystalline silica has various shapes. If you go to have a look at the crystals in Hainan Crystal Gallery, you will be stunned. Besides long cylindrical crystals, a sword-shaped, plate-shaped, short-cylindrical and biconical crystals can be seen. Among them, some are as small as a finger, some are as large as a huge stone; some are less than half of a liang and some are heavier than 300 kilograms.

Why can crystal stand general beats? Because it has a strong “bones”, which is called hardness by gemmology, indicating the capacity to resist scrapes.

How to measure the hardness of crystal?
Long long ago, some pawnshops bosses give those suspicious imitation crystals hardness tests by carving with a harp tip or steel file, and they leave behind some unconspicuous traces to remind other pawnshop peers of paying attention. Obviously, it is one of the most original and clumsy testing methods, which could lead to break crystals.
In 1824, an Austrian mineralogist named Frish Moss extracted 10 varieties from lots of minerals and measure their relative hardness through scientific experiments, resulting that the hardness of crystal is Mohs hardness of 7. Later, Knoop hardness tester is used and promoted by US National Bureau of Standards, but many jewelers over the world are still accustomed to Mohs hardness table.

The hardness of crystal is equivalent to that of steel files, which makes those precious stones such as topaz, diamond, opal stone and emerald dwarfs.

Crystal cleavage: None.
Cleavage, commonly known as split. Some certain precious stones(such as topaz, diamond) will be split down one plane after the external beat, squeeze or collision. The nature is called cleavage.

Crystal break: conchoid.
Break, known as fracture according to mineralogy. It refers that precious stones will form a cut plane after the collision of external forces instead of rupturing down a certain crystallization direction. For an interpretation is that precious stones can rip down an extension in any direction, rather down the direction of cleavage.
Crystallization of intact crystal has often good chevron fractures of parallel-ridge-shape; the breaks show the shape of a non-flat thin sheet in amethyst and heat-treated citrine.

Crystal share: 2.56-2.66grams/cubic centimetres.
It means that the weight of a certain volume of crystal is 2.56-2.66 times heavier than the same volume of water. Massive crystal variants are probably slightly denser.

Crystal Streak: colorless.
Streak, usually the color, is the result of human’s eyes feeling the wavelenghth of light.
Crystal transparency is related to the quality and quantity of its light. Criteria of transparency: when the light penetrates through the crystal fragments and flakes with the thickness of 1 cm above, you can see the reflected images clearly. If the image cannot be seen clearly but only an outline, which is semi-transparent.
Crystal gloss: shiny glass. It is so not only on the polished surface but also in the break.
Gloss indicates an optical nature reflected by the surface of the gem. Crystal dosen’t give a beautiful reflection of the star-shaped stripes like star sapphires, a light blue glory like moonstones and a colorful flash like opal stones.
You can observe crystal sheeen by holding it and watching the reflection on surface with light and the light coming in windows. The brightness of transparent crystals is related to whether the luster is strong or weak.

Crystal refractive index: 1.544-1.553, hardly beyond the range.
Refractive index indicates the sine ratio between the angle of incidence and rafractive angle. When the light through air penetrates crystal stone, the phenomenon of refraction occurs, producing the angle of incidence and refractive angle.

Crystal re-refractive index: 0.009 (maximum), this value is very stable.

Crystal optical characteristics: 1 Zhengguang of equiaxed grains.

Crystal dispersion: 0.013.
Dispersion means that the refractive index of the gem changes with the changes of different lights. For example, the refractive index of dimonds to red light is 2.405; to green light is 2.427; to purple light is 2.449.

Crystal melting point is 1713 degree Celsius.
Its fragile thermal characteristics are found in the experiment. If crystal is burned on the spraying flames, it will crack easily unless with well protection and slow cooling. The ancients had mastered the temperament. According to The General Introduction to Natural Things, “Crystal will be cracked into powder with hot water injecting it.”
It is another temperament that crystal can touch acid (except acid gas) but not alkali, which is decided by the characteristics of silicon oxide.

When the precious stones like agate are heated, with friction, blown or beated, they will throw off a special smell, which reminds human of garlic, horse meat, radish and rosin. However, crystal give no smell in the circumstances above.

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The chemical composition and nature of crystal:
Chemical formula is SiO2. Pure colorless transparent crystal is a variant of quartz. Chemical coposition contains Si—46.7％,O—53.3％. It shows a variety of color because of different mixtures or mechanical mixtures. Purple and green are from iron (Fe2+), and purple can be induced by titanium (Ti4+). Other colors are from the center of color. Crystal contains those wrapping things of sandy, debris-like goethite, hematite, rutile, magnetite, garnet, chlorite, etc.; hair crystal contains the wrapping things of hair-like and needle-like minerals which can be seen by naked eyes. Amethyst contains manganese and iron; yellow crystal contains iron(golden yellow or lemon); rose quartz contains manganese and titanium with rosiness; smoke colored crystals are cairngorm; brown crystals are called citrine; trasparent black crystals are called inky crystal; light green stones are called chalcedonic.

Crystal structure and crystal morphology:
A trigonal system. Crystal is with hexagonal prism-shaped and cone, and cross striations on the surface of cylinder. There is often horny ribbon in amethyst. In nature, crystal is often shaped in crystal cluster, handsome in appearance.

The physical properties of crystal:
Crystal is colorless, purple, yellow, green, smoke color, etc. Glass luster. Transparent to translucent. Hardness of 7. And brittle. Proportion of 2.65. No cleavage. Conchoidal fracture and equal cheveron ridge-shaped fracture. Amethyst has a clear dichroism, while yellow crystal and citrine are with a weak dichroism. Crystal is strongly phosphorescent. Aventurine crystal with green gives a celadon fluorescence in the long and short ultraviolet radiations with the effect of opal, iridescence and golden dust.