Jargon Gemstone | Globalgemology

LANGUAGE ELEMENTS

There are dozen of extremely well built website made by gemmologists for specialised people. At GIGA we do not think, you need to be an expert to chose your gemstone. Every gem on our website come with a description of its physical properties. Here are the defintion of the words we use.
If you want to know more about the tools gemologists use, visit:
www.globalgemologytools.org

DISPERSION

The amount of fire is a function:

Facet angles
Polish quality
Lighting environment
Gemstone refractive index
Orientation of the viewer relative to the gemstone.
Gems with a darker body color will exhibit less fire than lighter colored gems with very good clarity.

Dispersion occurs when white light is separated into colour according to wavelength

MAXIMUM DISPERSION IN A WELL CUT DIAMOND

INCREASING THE DISPERSION BY CUTTING A CRISTAL

SPHENE CRYSTAL: LOW DISPERSION

CUTS FROM THE SAME CRYSTAL: HIGH DISPERSION

IMPROVING THE DISPERSION BY IMPROVING THE CUT

POORLY CUT GARNET: LOW DISPERSION

IMPROVED CUT ON THE SAME GARNET: HIGH DISPERSION

SOME VERY HIGH DISPERSION

MOISSANITE

0.104

SPHENE

0.051

SPHALERITE

0.156

FROM LOW TO HIGH DISPERSION IN MOST COMMON GEMSTONES

FRACTURE

Fracture is the characteristic mark left when a mineral chips or breaks

Obisidian

Conchoidal

Fracture resembling a semicircular shell, with a smooth, curved surface

Magnetite

Uneven

Fracture that leaves a rough or irregular surface.

Nickel

Hackly (jagged)

Fracture that resembles broken metal, with rough, jagged, points.

Chrysolite

Splintery

Fracture that forms elongated splinters. All fibrous minerals fall into this category

Limonite

Crumbly (Earthy)

racture of mineral that crumbles when broken

Titanite

Subconchoidal

Between conchoidal and even

LUSTER

Luster is the general appearance of its surface in reflected light.

Microcline feldspar

Dull luster

Aurichalcite

Vitrous luster

Actinolite

Silky luster

Pyrite

Metallic luster

Azurite

Earthy luster

Chrysocolla

Greasy luster

Amber

Resinous Luster

Chalcedony

Waxy luster

Barite

Pearly luster

Herkimer Diamond Quartz

Adamantine luster

CLEAVAGE

Cleavage is the tendency of a crystal to break cleanly along distinct planes.
Most gemstones are crystals. Cleavage is an issue that gem buyers as well as jewelers and gem cutters need to take into account.
Minerals cleave along particular crystallographic planes where the atomic bonding is weaker. (similar to splitting a piece of wood - it splits fairly easily along the grain, but not across the grain)
Number of cleavage planes and the angles between them are characteristic of specific minerals.
Minerals can have from 1 to 5 cleavage planes.
Each cleavage plane has a grade or rating (indicating the relative ease with which the crystal can be cleaved)

Cleavage can be NONE, POOR, GOOD or PERFECT

Uneven surface when breaking

Ex. Quartz No Cleavage

Basal

One direction

Ex: Muscovite

Basal Cleavage : exhibited on a horizontal plane of the mineral by way of its base. Minerals with basal cleavage can sometimes be "peeled"

Prismatic

Tow direction

Ex: Orthoclase

Prismatic cleavage on some prismatic mine-rals in which a crystal cleaves as thin. verti-cal, prismatic crystals off of the original prism.

Cubic

Three direction

Ex: Halite

Cubic cleavage exhibited on minerals of the isometric crystal system that are crystallized as cubes. In this method of cleavage, small cubes evenly break off of an existing cube.