Gem properties

Introduction

Majority of natural gems come from minerals (natural substance of chemical elements that form the Earth). Can be hard, transparent, colourful & rare = perfect material for gems. Some minerals, before cutting form beautiful crystals which in themselves are collectable and valuable. Many minerals but only some form good crystals depending on the environment they grew in.

There is a good market to collect rare mineral specimens - may not be large enough to be cut or too difficult to cut.

What is a crystal:

• Raw material for gemstones.
• About 5700 distince mineral species on earth but onlu c.200 are of the required quality and size to be cut into gemstones.
• The crystal of gem material is not the same as the word 'crystal' which can be applied to cut glass.
• Crystals of some species will always have the same internal structure even if they appear as different shapes. Atoms come together under specific geochemical and geological conditions. Need to precipitate from the solution into the crystal, over time. Stability of an environment allows them to grow large. Need the right temperature, right chemicals and right amount of space to grow into. It's a closed system - once its crystallised, that's it. But if have environments where fluids can pass through cavities then crystals can get bigger until the fluid runs out.

Internal symmetry:

• eg: rubies and sapp look different but the crystals and 6 sides eg: hexagonal bi-pyrimid of a sapphire.
• eg: quartz varieties differ in size and colour but arrangement of atoms remains the same. One of the most common minerals.
• Crystals are solids with regular flat surfaces/faces that directly reflect the internal arrangement of atoms within - so angles between similar faces remains the same (with experience can ID a mineral by the external shape of the crystal).

Different in form / how hard / how light passes through them. Properties governed by the arrangement of atoms. By observing and measuring gemm'ts can ID.

Hardness

The degree of resistance to scratching; scale used is Mohs' (relative hardness); if too soft then will scratch when worn as jewellery. Gemm'ts avoid scratch testing as it is destructive but in observation, scratches give a clue to hardness and ID, or age, if it's worn well. Mohs:

• 1 talc
• 2 gypsum
• 3 calcite
• 4 fluorite
• 5 apatite
• 6 orthoclase
• 7 quartz
• 8 topaz
• 9 corundum
• 10 diamond

Toughness: is different. Being tough is being resistant to breaks of chips even though they may be easy to scratch. Eg: dia. is hard but not tough as can be smashed if hit the right way with a hammer (don't store dia. with other gems as they can scratch them). Eg: citrine can shatter if dropped and cultured pearls surface can pit with vinegar.

Fracture: the way gems break eg: shell-like as seen on broken glass, is called 'conchoidal'. Ancient Greek for shell. Seen on a variety of gems such as quartz, emerald, tourmaline and glass imitations of gems.

Cleavage

• is a more defined break - the tendency for some gems to split in specific directions relating to their crystal structure, like wood splits along the grain.
• diamond can be deliberately cleaved into two or more prior to cutting, this is a risky process made easier now with laser cutting. Dia. can cleave parallel to its edges.
• sometimes can observe something like a planar surface passing through a crystal, often glassy smooth. This feature can help ID eg: mineral calcite or topaz.
• gems can cleave due to excessive heat or pressure when being worked on. Care must be taken when setting gems or repairing jewellery.

Dimensions & Weight

These affect importance and value of a stone. If you have a similar, good quality gem then the larger will be worth more.

Most of the world uses grams as the measurement, but the USA uses pounds/ounces. Larger pieces of gem material are weighed in kilos (or pounds), not carats. Crystals of cut gem material and cut & polished gems are weighed in carots. This is an ancient unit derived from Arabic 'qirat' (little horn) and relates to the shape of the carob tree pods. These have uniform seeds which were used as a standard weight.

Carat is an international unit of weight for gemstones. Gold can also be described as eg: 18 carat or pure 24 carat. This use of carat unit goes back to Constantine the Great (272 - 337 CE) who coined the solidus which weighed 24 carob tree seed of pure gold. The other carat value of gold we see today is an expression of how it relates to the 24 carat pure standard, being proportionally used to describe fineness of gold alloys. The standard high jewellery gold has 750 parts of pure gold per 1000 = 750/1000 x 24 = 18 (giving 18 carat gold alloy). Use of 'k'or 'kt' will relate to the context being used - carat for gems and karat for gold.

1 carat = 0.2gms (also = 100 points). A carat is divided into 4 grains or given as a fraction or as a decimal (expressed to the 2nd decimal place). Eg: emerald described as 4 1/4 carat or 4.25 carat but not 4 carot and one grain:

• grain 1 = 1/4 carat or 0.25
• grain 2 = 1/2 carat or 0.5
• grain 3 = 3/4 carat or 0.75
• grain 4 = 1/1 carat or 1.00

The price of a cut gem is often expressed as U$ / carat eg: a 5.00 carat stone at $50 per carat = $250.00.

Different gem types of the same dimension may differ in weight eg: dia. weighs more than glass. Gemm'ts will gain useful info by 'hefting' a gem (comparative weight in hand / hold & feel weight). Specific gravity (described in relation to something being heavier than water) is an important physical property and can be roughly estimated through heft. Some gem material is denser and will fell heavier eg: diamond (spec grav = 3.52) and cubic zirconia (spec gravity = 5.8) so is c.60% heavier than a diamond.

Light

[see also Gem observation for a section on Light]

Transparent - light travels through gem / Translucent - Some light travels through / Opaque - no light passes through.

Lustre: relates to quality and quantity of light reflected off a gem. Not affected by transparency or any internal features. Has distinct terminology:

• vitreous - seen in reflections of window or bottle glass; seen in rough & polished gemstones like emerald, agate, tourmaline.
• bright vitreous - even brighter than polished glass eg: ruby & sapphire.
• adamantine - very bright, mirror-like . Generally only used with diamonds.
• sub-adamantine - not quite as bright as adamantine but bright & glossy eg: zircon & demantoid.
• mettalic - high lustre seen on polished gold & polished hematite.
• others - dull, arthy, silky, waxy, pearly