Jargon buster

The following are explanations of a number of terms used on the 'Scientific facts' pages. We hope that these explanations will help in your understanding and appreciation of the various minerals and gems.

Adamantine - Used to describe Lustre (see below). Adamantine minerals possess a superlative lustre, which is most notably seen in diamond. Such minerals are transparent or translucent, and have a high refractive index

Cleavage - In mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the microscope and to the naked eye.

Crystal Habit - In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals.

Crystal System - A crystal system or crystal family or lattice system is one of six or seven categories of space groups, lattices, point groups, or crystals. Informally, two physical crystals tend to be in the same crystal system if they have the same symmetries, though there are many exceptions to this.

Density - The density of a material is defined as its mass per unit volume.

Fracture - In the field of mineralogy, fracture is a term used to describe the shape and texture of the surface formed when a mineral is broken. Minerals often have a highly distinctive fracture, making it a principal feature used in their identification. Fracture differs from cleavage in that that the latter involves clean splitting along the cleavage planes of the mineral's crystal structure, as opposed to more general breakage. All minerals exhibit fracture, but when very strong cleavage is present, it can be difficult to see.

Lustre - Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. For example, a diamond is said to have an adamantine lustre and pyrite is said to have a metallic lustre. The term is also used to describe other items with a particular sheen (for example, fabric, especially silk and satin, or metals). The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance.

Mohs scale - The Mohs scale of mineral hardness characterises the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is one of several definitions of hardness.

Paramagnetic - Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field.

Specific Gravity - Specific gravity is defined as the ratio of the density of a given solid or liquid substance to the density of water at a specific temperature and pressure, typically at 4 °C (39 °F) and 1 atm (760.00 mmHg).

Streak - The streak (also called powder colour) of a mineral is the colour of the powder produced when it is dragged across an unweathered surface. Unlike the apparent colour of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic colour, and is thus an important diagnostic tool in mineral identification. If no streak seems to be made, the mineral's streak is said to be white or colourless. Streak is particularly important as a diagnostic for opaque and coloured materials. It is less useful for silicate minerals, most of which have a white streak and are too hard to powder easily.

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