CRYSTAL STRUCTURE

Each gemstone group or variety has a unique chemical composition that describes its essential elements or ingredients. Each gemstone is made up of atoms, the smallest building blocks of the structure. Where the atoms are constituted in a regular and repeating three-dimensional pattern, they are said to possess crystal structure and are referred to as crystalline.

amorphous

Where there is no crystal structure and atoms are randomly arranged, the material is said to be amorphous (from amorph meaning without shape or without form). obsidian (volcanic glass) is an example of an amorphous material that can be used as a gemstone.

crystalline

Most natural and artificial gemstones are crystalline. Crystalline materials have directional properties, both physical and optical, that are as a result of the three-dimensional arrangement of atoms and the type of bonds between them. As the gemstone forms, it grows by adding layers of atoms. The many ways that the atoms can com­bine is reflected in the many different crystal shapes including those with flat faces and sharp edges, needle-like points, or with smooth and rounded surfaces.

Although the internal crystal structure of the crystalline gemstone may define its external shape (habit), this is not always the case. Con­ditions during formation may be such that an irregular shaped crystal forms, or a crystal may have been broken, cut or polished. Whatever the final shape of the crystal, the internal structure is still the same, it is still crystalline.

twinning

The crystal structure may have parts that are reflected, repeated incorrectly or rotated, which may result in the creation of a twin crystal. At the junction of the parts of the twin, the change in orientation of the crystal structure (lattice) and direction of growth may be seen as a re-entrant angle at the surface. Repeated twinning (lamellar twinning) can sometimes be seen as layers in gemstones. Twinning of a crystal structure may be due to changes in temperature or pressure during or after formation.

POLYMORPH

Occasionally the chemical composition is such that it can form more than one crystal structure. Each structure is called a polymorph (from 'poly meaning 'many' or 'more than one', and morph meaning 'shape'). Which of the polymorphs forms will depend upon factors such as conditions during formation (for instance, temperature or pressure)? diamond and graphite, for example, are formed from the element carbon, but the carbon atoms have bonded in different ways, forming different crystal structures in each of the minerals.

ISOMORPH

Some gemstones have a range of chemical compositions, but have the same crystal structure. They are called isomorphs. An example is the garnet group. At opposite ends of the range are the two end members and between these are the members of the isomorphous series. almandine and pyrope are respectively the- iron-rich (Fe) and the magnesium-rich (Mg) end members of the garnet family.

cryptocrystalline

Crystals composed of many crystal structures that have grown together are termed polycrystalline. Where the crystal structure is too small to be seen with the eye and a microscope is needed, the crystals are termed microcrystalline or cryptocrystalline (from the Greek crypto meaning 'hidden').

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