Minerals can appear in nature, basically, in two ways: without a definite shape (amorphous) or with a well-defined geometric arrangement. We call these crystalline minerals or simply crystals.
For a place to form crystals, space is needed. Because of that, they usually appear in cracks or empty cavities in the rocks. They also appear as part of soft rocks, which facilitate their growth.
Many minerals adopt crystalline forms when the formation conditions are favorable. Crystallography is the study of the growth, shape and geometric character of crystals. The arrangement of atoms in a crystal can be determined by means of X-ray diffraction analysis. Crystallographic chemistry studies the relationship between chemical composition, the arrangement of atoms and the bonding forces between them.
Most of the earth's crystals formed millions of years ago. Crystals form when the liquid rock inside the Earth cools and hardens. Sometimes crystals form when underground liquids travel their way through the cracks and slowly deposit minerals.
There are many crystals that react to a physical action differently depending on the direction in which the force is produced. They are called crystals anisotropic. Amorphous minerals, on the other hand, react to a physical action always in the same way, regardless of the direction, for those isotropes.
Law of constancy of dihedral angles
When the temperature conditions are the same, crystals of the same type have the same dihedral angles.
Gems or precious stones
They are called various hard, transparent minerals, very valuable for their rarity and which, after having been conveniently carved, are used in jewelry and decorative arts. Two types are usually distinguished:
Precious stones, considered luxury objects since ancient times: diamond, ruby, emerald, sapphire, ...
Fine stones, whose price in the market is not so high: topaz, amethyst, garnet, tourmaline, ...
Science, using increasingly sensitive analytical means, is discovering the substances that color the allochromatic minerals. Thus the amethyst has a violet color due to traces of manganese and the fluorine is green because of very small amounts of iron and manganese it contains.
The beauty of gems depends largely on their optical properties. The most important are the degree of refraction and color. Other properties include: fire, display of prismatic colors; Dichroism, the ability of some stones to show two different colors depending on the direction in which they are observed, and transparency.
The diamond is much appreciated for its fire and shine; the ruby and the emerald for the intensity and beauty of its colors; and starry sapphire stands out for its color as well as for asterism, a property that causes the appearance of star-shaped inclusions.
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