Gemstone Formation : Gemstone Hardness Table

Gemstone Formation
As, with a few exceptions, most gemstones are minerals, we must concern ourselves with the origin and structure of these minerals. The formation of the non-mineral gemstones (for instance amber, coral and pearl) will be dealt with in more detail when they are described.

Minerals can be formed in various ways. Some have their origin in the magma and fiery gases of the earth's interior, or in the volcanic lava streams which reach the earth's surface (magmatic minerals). Others crystallize from hydrous solutions or grow with the help of organisms on or near the earth's surface (sedimentary minerals). Lastly, new minerals are formed by re- crystallization of existing minerals under great pressure and high temperatures in the lower regions of the earth's crust (metamorphic minerals).

The chemical composition of the minerals is shown by a formula. Impurities are not included in this formula, even where they cause part or total colour change, as with pigmenting substances.

Nearly all minerals grow in certain crystal forms, i.e. they are homogeneous bodies with a regular lattice of atoms, ions and molecules. They are geo- metrically arranged and their outer shapes are limited mainly by flat surfaces (faces).

Most crystals are small, sometimes even microscopically small, but there are also some giant specimens. Such large minerals are usually unsuitable as gems as they normally have numerous inclusions, impurities or various growth marks.

The inner structure, the lattice, determines the physical properties of the crystal: its outer shape, hardness, cleavage, type of fracture, specific gravity and optical properties.

In crystallography, crystals are divided into seven systems (isometric (cubic), tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic).

Isometric system (regular and cubic) All three axes have the same length and intersect at right angles. Typical crystal shapes are the cube and octahedron (8 faces), rhombic dodecahedron (12 square faces), pentagon dodecahedron (12 pentagon faces), icosi-tetrahedron (24 faces) and hexoctahedron (48 faces).

Tetragonal system (four-sided or tetragonal) All three axes intersect at right angles, two are of the same length and are in the same plane while the main axis is either longer or shorter. Typical crystal shapes are four-sided prisms and pyramids, trapezohedrons and eight-sided pyramids as well as double pyramids.

Hexagonal system (six-sided) Three of the four axes are in one plane, are of the same length and intersect each other at an angle of 120' (respectively 60'). The fourth axis is at right angles to the others. Typical crystal shapes are hexagonal prisms and dipyramids as well as dihexagonal dipyramids and double pyramids.

Orthorhombic system (orthorhombic or lozenge-shaped) The three axes of different length are at right angles to each other. Typical crystal shapes are basal pinacoids, rhombic prisms and pyramids as well as rhombic double pyramids.

Monoclinic system (singly inclined) The three axes are each of different lengths, two are at right angles to each other, the third is inclined. Typical crystal forms are basal pinacoids and prisms with inclined end faces.

Triclinic system (thrice inclined) All three axes are of different length and inclined to each other. Typical crystal forms are paired faces.

Most crystals are not regularly shaped, but have an irregular form, because some crystal faces have developed better and are more pronounced than others; however the angle between the faces always remains constant. Many minerals occur in a combination of various crystal forms, for instance, octahedron and cube. 80 such combinations are known in calcite.

The arrangement of faces preferred by a mineral is called "habit", for instance pyrite is found in the shape of a pentagon dodecahedron, garnet as a dodecahedron. The habit of a crystal also refers to its type and can be tabular, acicular, foliated, columnar or compact. The technical terms, habit and form, are sometimes called structure for the benefit of the layman.

Sometimes minerals occur in unusual, for them, crystal forms. These are called pseudomorphs. They can originate in two ways: they either take up the space vacated by a dissolved mineral or they form a crust around a foreign crystal.

Where two or more crystals are intergrown according to certain laws, one speaks of twins, triplets or quadruplets. Depending on whether the individual crystals are grown together or intergrown, one speaks of contact twins or penetration twins. Triclinic feldspars often occur as contact twins in a foliated form. The lamellae look like striations.

Apart from twinning which adheres to certain laws, many crystals are irregularly intergrown into aggregates. Depending on the growth process, filiform (wire-like), fibrous, shell-like, scaly or grainy aggregates are formed.

Well developed, characteristic minerals are formed as druses on the inner walls of rock openings (geodes); these are mainly round hollows created by gas bubbles in magmatic rocks. The best known drusy mineral is amethyst.

Seemingly structureless minerals are called compact. These have a crystal- line lattice, but are irregularly grown due to growth restrictions.

Gemstone Hardness Table

Scratch Mineral Simple Hardness Tester - (Mohs) comparison