What does all the Scientific Jargon Mean?

Hardness

Refers to the Moh’s Scale.

Cleavage

The cleavage of a gemstone is the tendency of a crystal to break along definite plane surfaces. If there is a crystal structure with weak atomic bonds, then the crystal is more likely to break along its planes. For example, when you look at clear Selenite, you can see what appear to be thick layers, these are the planes. If you were to crack the stone at one of those ‘layers’, it will easily split apart as one big slice, rather than into many smaller pieces. As opposed to the glass where if you tried to break it, it would just shatter into many pieces.

There are 5 degrees of cleavage (none, poor (or weak), fair (or moderate), Good (or imperfect) and perfect) - where perfect is the easiest to split - and it is usually described in relation to crystallographic axes and directions and is graded according to the ease with which the mineral can be split along the cleavage.

So if it is said that stone has cleavage of ‘2 directions, good’ then it means there is cleavage in 2 different directions and it is relatively easy to split.

What does it mean for turning the stone into jewellery?

Stones with good cleavage are typically a nightmare to facet into jewellery worthy pieces and therefore must be set and worn carefully as a sharp blow along a cleavage line could easily split it.

Fracture:

This is the way that a mineral breaks other than along the cleavage directions.

There are 4 ways that this is usually described:

Conchoidal fracture

This is shell-like and has concentric curved lines. Think of how a chunk of glass looks when it breaks.

Fibrous/Splintery Fracture

This is used to denote a fracture that has a sharp and elongated point.

Hackly Fracture

This produces sharp and jagged points

Uneven Fracture

This produces a rough and uneven surface that doesn’t have sharp points.

Parting

This refers to the breakage of a mineral along a structural weakness (such as twinning (explanation below)

Parting isn’t present in all specimens of any given mineral but is instead a result of the growth of each individual specimen.

Twinning

This occurs when 2 or more adjacent crystals of the same mineral form so that they share some of the same points in a symmetrical manner, resulting in 2 separate crystals growing together in such a manner that they cannot be separated.

Crystallography

This is the study of the formation and structure of crystalline solids and this term is used to help visualize how minerals have developed.

The arrays of the crystalline structure are classified into 6 major crystal systems and are defined in terms of crystal axes and angles.

• Isometric

• Tetragonal

• 

  Orthorhombic

• 

  Monoclinic

• 

  Triclinic

- Hexagonal

Crystal Axes

These are imaginary lines in space bewteen the sides of the crystals that all intersect at a common point with equal or unequal lengths to each other.

They intersect each other at various angles.

Refractive Index (RI)

This is one of the most important signatures of a gemstone and is one of the first steps of identification.

It is the difference between the speed of light in a vacuum and the speed of light in a gemstone. As a gemstone is denser than air, the speed of light passing through it slows down causing the path of light to change.

Refraction is the bending of the light as it passes in or out of a gem and the angle of refraction in the stone determines its Reflective Index (RI).

What does it mean though?

If a gemstone has a high RI, then it is more brilliant that those with a low RI. For example, Diamond is more brilliant with an RI of 2.42 than quartz with an RI of 1.54-1.55

Birefringence

Some gemstones can polarize light and split it into 2 or 3 different directions and are said to be doubly refractive. Birefringence is the measurement of a gem's double refraction, serving as another identification method.

Gems with an isometric or cubic crystal system (such as diamonds) and amphorous gems (like opals) have only 1 RI and no birefringence, however, gemstones with all other crystal system do have birefringence and have 2 or three RI’s.

The birefringence number is the difference between the mineral's highest and lowest RI’s and the greater the number, the more noticeable the effects of the double refraction will be.

How can this appear to the naked eye?

Birefringence can appear as:

-Pleochroism (image is of tanzanite)

This means “more colours”. Pleochric gems show 2 (dichroic) or 3 (triochroic) colors when viewed from different angles (such as in emerald and tanzanite).

- Fuzziness (gives an out of focus appearance)

- Double images (like double vision) and some will create a double image of whatever is behind it (such as some calcites)

Dispersion

Gemstone dispersion is when you see the colours of the rainbow coming from deep within a finely cut gem.

This can be measured by using a red light and then a violet light instead of a yellow one to measure the gemstone's RIs then taking the difference between the red and the violet light to equal its dispersion.

What does the dispersion number tell us?

The higher a gems dispersion value, the more colourful the flashes will be. The higher the number, the better the dispersion value. For example, Diamond has a dispersion of 0.044, Cerussite is 0.055 and Rutile is 0.28, meaning rutile has a much more colorful display than diamond or cerussite.

HOWEVER, not every specimen of each mineral will have the exact same dispersion and show the same amount of fire. The density, colour and RI all affect the dispersion of the stone which is when faceting plays a critical role in how the dispersion is shown and skilled lapidaries can control how the light moves.

Luminescence

This is when a gemstone appears to glow.

Absorption Spectrum

This tells us how a stone interacts with light and gets its colour. Determining which wavelengths of light that gemstone absorbs, can help us identify what stone it is and can also help identify whether it is natural or synthetic, or if it has been subjected to colour treatments.

Optics

This is very useful for identifying and distinguishing gem species from each other and it takes all sorts of information gathered on the light movement through the crystal into account.

Gems with a single RI are classified as singly refractive (SR) and when light enters them it only slows and bends. In DR (Doubly refractive) gems (those with 2 RIs), light is also polarised, splitting into separate rays.

The greek letters (alpha, beta and gamma) designate the refractive indices of a biaxial crystal.

Alpha refers to direction X with the fastest light speed within the crystal and has the lowest RI.

Beta refers to the Y crystallographic direction and represents an intermediate ray velocity and has an intermediate RI.

Gamma corresponds to the Z crystallographic direction and has the slowest ray velocity but the highest RI.

Lustre

This is whether the stone is shiny or dull.

Specific Gravity

This is referring to the density of the gemstone and is a way of differentiating between 2 similar-looking minerals. For example, Ruby and Garnet have the same dimensions and look similar but have 2 unique densities (or specific gravity).

Specific gravity is the ratio expressing the weight of a given material compared to that of an equal volume of water at 4 degrees Celcius. So this means that specific gravity of 3 means that one cubic centimetre of the material in question weighs 3 times that of one cubic centimetre of water.

Chatoyancy

Chatoyancy is another word for the cat-eye effect that occurs in stones like Tigers Eye.

https://www.gemselect.com/gem-info/refractive-index.php

https://www.gemsociety.org/

Images: Google