Properties of Minerals

Crystals and minerals, and particularly gemstones, are identified through several means. You may not use these these diagnostic methods, but I believe it is useful to have an idea how the properties are determined. These properties are listed in most rock and mineral books, and are helpful in recognition.

 

Crystal Form and Habit

A combination of the chemical composition and inner structure of a mineral will determine its properties. Many minerals are related, either because they possess the same chemical compound, or are of the same type of crystal structure. The properties of a mineral will include the outer shape, hardness, cleavage, optical qualities, type of fracture and specific gravity.

Fibrous Radiating Crystals displayed in Aurichalcite

The term habit refers to the arrangement of faces preferred by a mineral. This can also refer to its type, and can include descriptions such as long, short, fibrous, needle-shaped, prismatic, equidimensional (containing crystals of roughly equal size), columnar, tabular, or compact.

Botryoidal Goethite

The term aggregate refers to an assemblage of crystals. An aggregate is considered to be granular if the crystals within it are equidimensional, and granular aggregates can further be described as fine, medium or course-grained, according to the size of the crystals. Aggregates are also described as scaly, hair-like, foliated, radiating, columnar, or wire-like, and also dense, massive, banded, stalactitic (formed as stalactites and stalagmites), botryoidal (shaped like a bunch of grapes), reniform  (kidney-shaped), oolitic (small spheres), or pisolitic (slightly larger pea-sized spheres). In addition, some are described by the terms dendritic (moss-like appearance, although the root of the word comes from the Latin for tree), and arborescent (shaped like tree branches.) The distinction between crystal habit and shape is somewhat vague.

 

Hardness

Hardness is gauged using a scale to determine the hardness of minerals, from one to ten, one being the softest and ten the hardest. Knowing the hardness of gemstones and minerals is of particular importance to the lapidarist or jeweller who will be working with the stone. It is also of importance in recognition, but for crystal healers, it is beneficial to know hardness in order to take proper care of the minerals you are working with. If a stone is soft, it will be easy to scratch, and therefore should not be carried unprotected with other stones.

The term hardness refers first to scratch hardness, and then to cutting resistance.

Scratch Hardness
Scratch hardness is the resistance of a mineral when scratched with a pointed testing object. The Viennese mineralogist, Fredrich Moh (1773-1839) set up a comparison scale using ten minerals exhibiting different degrees of hardness. This scale is still in use today, and is known as Moh’s Hardness Scale. Gemstones of the hardness 1-2 are considered soft, 3-5 medium hard, and those over 6 hard (formerly this was 8-10, but has since been changed to incorporate other commonly used gemstones). Those gemstones graded below 7 can be scratched with dust, which may contain particles of quartz, and these stones should be carefully handled to avoid scratching and the resulting dullness of lustre.

Cutting Resistance
This is of more importance to the gemstone cutter. In some stones, the hardness varies according to the direction of the cut. For example, the hardness of kyanite along the length of the crystal is 4 – 4.5, but when cutting across the crystal, the hardness is 6 – 7.

Scratch Harness

Mineral Used for Comparison

Simple Hardness Tester

Cutting Resistance

1

Talc

Can be scratched with fingernail

0.03

2

Gypsum

Can be scratched with fingernail

1.25

3

Calcite

Can be scratched with copper coin

4.5

4

Fluorite

Easily scratched with knife

5.0

5

Apatite

Can be scratched with knife

6.5

6

Orthoclase

Can be scratched with steel knife

37

7

Quartz

Scratches window glass

120

8

Topaz

175

9

Corundum

1,000

10

Diamond

140,000

 

Cleavage and Fracture

Rose Quartz displaying Conchoidal Fracture

Fluorite Octahedron showing Perfect Cleavage

Many gemstones can be split along flat planes, known as cleavage planes. This is related to the lattice of the crystal (see the section on Crystal Systems). Cleavage refers to the ease with which a crystal can be cleaved, or split along a flat plane. If you’ve ever knocked your favourite fluorite pendant or ornament, and found it has broken along a straight line, you have witnessed cleavage. Most fluorite octahedrons are intentionally created in this way. Cleavage is usually listed in three categories: perfect (fluorite), good (sphene) and imperfect (peridot). Knowledge of cleavage is important for lapidarists, as the rise in temperature through soldering can create fissures along the cleavage planes, leaving the stone weak and easily broken. When faceting a stone with perfect cleavage, the facets must be transverse to the natural cleavage lines, or again the stone will be weakened and vulnerable. Cleavage is used to divide and break off faulty areas of large gemstones, although these days a saw is more frequently used.

Rose Quartz with Conchoidal Fracture

Fracture refers to the breaking of a stone with a blow, causing irregular surfaces, and the way in which these surfaces present can sometimes be used to identify a mineral. The categories for fracture are: conchoidal (or shell-like), uneven, smooth, fibrous, splintery or grainy. An example of fracture for identification is quartz and some other high-silica content minerals, which have a conchoidal fracture. Stone Age tools utilise quartz minerals or obsidian. The napping, or chipping away of material to create a sharp edge, demonstrates conchoidal fracture. Other categories are fairly self-explanatory.

 

 Density and Specific Gravity

In the past, the specific gravity of a mineral was measured by the ratio of its weight to the weight of the same volume of water. This has been replaced by density, usually expressed as grams per cubic centimetre (g/cm2). There are two methods of measuring the density of minerals: Hydrostatic Balance, using the buoyancy principal, and the Suspension Method, using a series of standardised heavy liquids (very expensive and potentially dangerous due to the toxicity of the heavy liquids). There are instructions for creating your own Hydrostatic Balance in Gemstones of the World by Walter Schumann, should you be interested.

 

Colour of Streak

Because one variety of crystal may occur in many different colours, caused by impurities, colour itself is not considered to be diagnostic. Streak is determined by “streaking” the mineral on a rough porcelain plate. The streak left will be the inherent colour of the mineral, which is constant.

 

Refraction

Refraction occurs when a ray of light leaves one medium, such as air, and enters another, such as water or a crystal, and the interface between these two media. The effect is that of bending the light, such as when a stick is partially immersed in water. The amount of refraction in crystals is constant, and can be used in identification. A good example of double refraction is optical calcite, in which half the incident light travels straight through the crystal, while the other half of the light is bent. If you place the crystal over a printed piece of paper, the image or writing will appear double.

 

Dispersion

Dispersion occurs when a colourless crystal disperses white light into its spectral colours, in the way that a prism does. This is especially notable in diamonds, and referred to as “fire”.

 

Absorption Spectra

Colour is one of the most important tools in identification, but it is not diagnostic, as numerous crystals are the same colour as others, and many also come in a wide range of different colours. Listing crystals according to colour, as some crystal healing books are prone to do, is not an efficient means of identification.

The Absorption Spectrum refers to the spectral colours of light as they emerge from a gemstone. Certain wavelengths, or colour bands, are absorbed, and the colour of the gem is formed from a mixture of the remaining parts of the original white light.

The two main causes of colouration in minerals are absorption of parts of the incident light, which affects the wavelengths of light leaving the mineral, and the refraction and scattering of light. When white light passes through a mineral, and none of the colour spectrum is absorbed, that mineral will appear to be colourless or white. If the entire colour spectrum is absorbed, the mineral will appear grey to black. If only certain wavelengths of colour are absorbed, the colour displayed by the mineral will be those wavelengths that pass through it. The wavelengths of light that are absorbed represent energy that has been expended moving electrons from one energy level to another, and the specific wavelengths required to do this will vary not only between different elements, but are also determined by the electron configuration of a particular element. When there is a transition of electrons from one level to another, this can cause a change of the wavelengths absorbed, even when the element is the same. An example of this is azurite and malachite, both of which are hydrous copper sulphates, but azurite is blue while malachite is green.

As you can see, the absorption of light is very complicated, and we will not go too deeply into it here. If you are interested in studying this further, more information can be found in Photographic Guide to Minerals of the World by Ole Johnsen (OUP), and Gemstones of the World by Walter Schumann (see Bibliography for details).

A simpler way of observing causes of colouration is to look at the colour range created by certain elements. Some minerals receive their colouration from the inclusion of impurities when the crystal is forming. In these cases the impurities are not included in the chemical formula, as they are not considered part of the actual make-up of the mineral. For example, all quartzes have the chemical formula SiO2 (silicon dioxide) whatever included elements are present, so clear quartz, amethyst, citrine, and even tiger’s eye and aventurine will be the same chemical formula. In other cases, the colour-causing element will be part of the chemical make-up, and therefore obvious in the chemical formula. It is important as a crystal healer to understand the chemical reasons for colouration, as this will have an effect on the vibrational qualities of the mineral. In crystal healing we use both colouration (as in Colour Therapy), as well as the vibrational qualities of the elements contained in the mineral.

Here are some of the most common colourations due to the presence of certain elements:

Aluminium: Blue, green (and grey)
Examples: Kyanite, variscite; small traces in smoky quartz will result in light absorption when exposed to gamma radiation or x-ray.
Chlorite:
Green
Examples: Bloodstone (colouration can also be from included hornblende), and in some green-coloured quartz phantoms
Cobalt:
Pink, silver
Examples: Cobaltocalcite; cobaltite
Copper:
The most common cause of green, turquoise and some blue colouration in minerals.
Examples: Malachite, turquoise, dioptase, chrysocolla, azurite
Chromium:
Pink to red; also green
Examples: Ruby; also emerald, jade, moss agate (which can also include hornblende), fuchsite mica, chromium diopside; alexandrite is between the two absorption spectra, and will appear red in artificial light, green in daylight.
Iron and Iron Oxide:
Blue to violet; yellow, orange and red; metallic silver to pewter
Examples: Amethyst, iolite, blue tiger’s eye, blue and yellow sapphire (blue sapphire also contains titanium), aquamarine; also citrine (due to heat and possibly gamma radiation); red jasper, gold tiger’s eye, carnelian, tangerine quartz, red jasper; also responsible for the pinkish tint on lemurian quartz and the red speckles in harlequin and strawberry quartz; hematite, magnetite (lodestone), goethite
Lithium:
Lavender to purple, mauve
Examples: Lithium quartz, lepidolite, sugilite
Manganese:
Pink
Examples: Rhodonite, Rhodochrosite, Mangano calcite
Nickel:
Green
Examples: Chrysoprase, Variscite
Titanium:
Pink, purple – blue
Examples: Rose quartz, sapphire (also contains iron)

Transparency
Transparency or clarity of a gemstone is a factor in grading and evaluating. Transparency will be affected by the inclusion of impurities, or fissures within the crystal. The fewer impurities or flaws, the greater the transparency.

Lustre
The characteristic lustre of many gemstones is used to help classify the stone, whether the stone is cut and polished, or a natural specimen. Lustre is not measurable, and is described with known objects, differentiated as metallic, diamond, greasy, pearly, silky, waxy, resinous and vitreous.

Pleochroism
Some minerals will display a different colour or depth of colour when observed at different angles, due to the differing absorption of light in double-refracting crystals. A classic example of pleochroism is Alexandrite, which appears green in natural daylight, and red in incandescent light, due to differences in light frequency and absorption.

Light and Colour Effects
This sub-section refers to light or colour effects that do not relate to a gemstone’s body colour, and are not caused by included impurities.

Adularescence: The play of light associated with moonstone (a feldspar), as a result of interference phenomena of the layered structure of the crystal.
Asterism
(from the Latin aster, meaning star): The six-pointed star effect displayed in minerals such as sapphire, ruby and rose quartz, when cut in a sphere or cabochon. This is usually caused by light reflecting off fibrous inclusions in the crystal, such as rutile.

Aventurescence in Aventurine Quartz

Aventurescence: The colourful and glittering play of light displayed in different types of aventurine, caused by small plate-like or leaf-like inclusions. The inclusions are hematite or goethite in aventurine feldspar, and hematite or fuchsite (mica) in aventurine quartz, the latter of which is the one most commonly called aventurine.
Chatoyancy
(from the French chat, or cat): Chatoyancy is also known as “cat’s eye effect”, and refers to a single line of light reflected from parallel needles, fibres or channels within the crystal. The most valuable cat’s eye occurs in chrysoberyl, but the effect also occurs in quartz and both blue and gold tiger’s eye..

Labradorescence in Labradorite

Labradorescence: Labradorescence is the metallic iridescent play of light that is usually observed in labradorite, hence the name. It is also known as schiller. The cause is not completely understood, but is probably due to lattice distortions combined with microscopic inclusion of other minerals.
Orient:
The iridescence in pearls.
Play-of-Colour:
The flash of rainbow colours that occurs in opal, caused by microscopic spheres of the mineral crystobalite included in a silica gel within the gemstone.
Luminescence:
Luminescence is the emission of visible light under the influence of specific light rays, most commonly ultraviolet, the effect of which is referred to as fluorescence. Fluorescence is not diagnostic, as specimens of the same gemstone will fluoresce in completely different colours, or not at all.

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