Crystallography

Crystal: It is a solid substance having a regular internal arrangement of atoms and a definite geometrical shape.

Amorphous: It is a solid substance having no internal arrangement of atoms and no definite shape.

Aggregate: Crystallization of a mineral from many different centres often produces a mass of interlocking anhedral crystals in random orientation.

Cryptocrystalline: The individual crystals of an aggregate material cannot be resolved by the naked eye but are visible only at higher magnification. Also known as microcrystalline, e.g. jade, chalcedony, turquoise.

Habit: This is a term used to define the general shape of a crystal, and is related to the length of the "c" crystallographic axis i.e. Long "c" axis (prismatic); short "c" axis (tabular).

Form: This is a set of identical faces, all of which have the same relation to the crystallographic axis. Those which have the ability to enclose space by themselves are called closed forms, while those which cannot enclose space by themselves are called open forms.

Metamict state: Owing to the effects of internal or external radioactivity, the crystal lattice becomes damaged, weakened and finally collapses due to the weakening and rupturing of the atomic bonds. This gives rise to an almost amorphous state which is known as the "Metamict State".

Crystallography is the science of crystals. The properties by which gemstones are identified are entirely based upon their basic crystal structure. Rough crystals themselves are very beautiful and well formed crystals display characteristic features such as cleavage, surface markings, forms etc. Hence, knowledge of crystals is absolutely essential. The fundamental principles are briefly explained below.

  • The formation of minerals, which ultimately form rocks, may be either fast or slow.
  • Slow formation results in well formed, large crystals provided other environmental conditions are conducive.
  • During slow formation atoms of different elements come together and arrange themselves in a pattern depending upon the forces acting on these atoms.
  • This regular arrangement of atoms in a definite orderly manner ultimately results in a framework or lattice of atoms.
  • When this frame work develops in a regular manner, it gives rise to a structure, which has smooth surfaces and geometrical form and is called a crystal.
  • The regular spacing of atoms within crystals also causes many differences in their properties including color, hardness, and ease of splitting etc., which a lapidary must take into account when cutting.
  • All natural gem materials, with the exception of pearl, coral, amber and a few others of organic origin are minerals.
  • When minerals or rocks do not have the regular internal arrangement of atoms as in crystals, they are called amorphous which means "no form" in Greek.
  • Here the atoms are mixed together in small groups like the particles in a pile of mud.
  • There is no tendency to take a definite form and consequently smooth crystal faces are never seen.

Difference between Crystal and Amorphous

  Crystal Amorphous
1. Definite geometrical shape. No definite geometrical shape.
2. An orderly and symmetrical internal arrangement of atoms. There is no regular internal arrangement of atoms.
3. Properties which vary with direction such as: hardness, cleavage, velocity of light etc. Their properties are constant in all directions.
4. Examples: Diamond, Quartz, Garnet, Zircon, Beryl, Chrysoberyl, Corundum etc. Examples: Amber (organic resin), Ivory (organic), Opal (silica gel), Pearl (partially crystalline / partially amorphous), Plastic, Glass (both natural and man-made); among the natural glasses are Moldavite, Obsidian and Tektites, while man-made glasses are known as "paste".
  1. Geometrical structure and appearance of crystal:
    • Minerals by definitions usually have a characteristic internal structure, which may or may not be expressed in typical outward forms called crystals.
    • The crystal form is the general equilibrium state of all solid substances.
    • Most minerals crystallize into distinct, geometrically regular, homogeneous bodies bounded by faces, edges " angles, with an ordered internal structure.
    • This internal homogeneity is retained even if the growing crystals come into contact with each other in the solidifying melt and their growth thus becomes irregular.

    The ability of assume a crystal shape is not exclusive to natural minerals because synthetic substances also form crystals.

  2. Internal Structure:
    • A molecule is the smallest possible unit cell of a compound that can exist independently and has the same properties as the compound.
    • A molecule is composed of a definite number of elements.
    • Each atom of an element is the smallest possible unit for that element and has a definite arrangement of sub-atomic particles like protons, neutrons, electrons etc.
    • The atoms of individual elements as well as atoms of different elements exercise forces very similar to the gravitational and magnetic forces.
    • Thus in every molecule there is a definite arrangement of atoms called the atomic lattice since the atoms and bonds, or imaginary lines linking them look very much like the lattice or framework.
    • This structure is also known as a crystal lattice or a space lattice.
    • Polymorphism: The arrangement of atoms in the structure may vary in substances with the same chemical compositions; e.g. Diamond and Graphite; the two crystalline forms of Carbon (C) have different internal structures.
      • In diamond the atoms are closely packed while in graphite the lattice has a much looser arrangement.
      • As a result, diamond and graphite have different crystal forms and different physical properties.
      • This is known as "Polymorphism" and diamond and graphite are said to be polymorphs of each other.
  3. Directional Properties: It is very interesting to note that, not only does the chemical composition play an important role to decide the properties of a mineral but also the arrangement of atoms inside the crystal affects the physical properties of minerals.

    • Depending upon variations in bond strengths, the forces vary in different directions.
    • When the forces of attraction are uniform in all three directions, then the physical and optical properties may remain isotropic.
    • The basic geometry of the smallest crystal unit tends to reflect itself in the finished crystal, which, no matter how large it grows, is nothing more than the neat stacking of the smaller units.
    • This is sometimes seen as distorted crystals depending on the growth rate of every face.
    • No matter what the final external appearance the internal angles between faces always remain constant. These angles are known as the interfacial angles.