Colour: Colour in minerals is caused by the absorption, or lack of absorption, of various wavelengths of light.
The colour of light is determined by its wavelength. When pure white light (containing all wavelengths of visible light) enters a crystal, some of the wavelengths might be absorbed while other wavelengths may be emitted. If this happens then the light that leaves the crystal will no longer be white but will have some colour.
Elements that produce colours through absorption and emission of wavelengths are usually transition metals.
They can cause a mineral to always be a certain colour if they are part of the chemistry of the mineral. These are the idiochromic minerals (Pict. 1.5.). However, if there is just a trace of these elements, they still can strongly influence the colour of the mineral. These are the allochromic minerals (Pict. 1.6.). Even tiny amounts of these elements can deeply collared minerals. It is erroneously thought that certain elements cause only certain colours and there is some truth to that. Copper usually produces green and blue colours. Iron is known for the red and yellow colours that it typically produces. However, almost any element can be responsible for any colour.
Pict. 1.5. Idiochromic mineral – malachite Pict. 1.6. Allochromic mineral – yellow quartz
Streak colour: Those minerals, although still subject to the effects of trace elements, always have the same basic colour. Most minerals, however, are usually white or colourless in a pure state. Many impurities can colour these minerals and make their colour variable. The property of streak often demonstrates the true or inherent colour of a mineral. In addition to colouring elements, other impurities or factors exist that have also been linked to the colour of minerals. Such things as elemental fluorine, sulphur, and chlorine; trace amounts of carbonate and other ion groups; chlorine and fluorine ions and even structural defects. Radiation from rare earth minerals can damage a crystal structure and this damage seems linked to colouring as in smoky quartz. Care should always be given when trying to identify a mineral using colour.
Lustre: The way a mineral transmits or reflects light is a diagnostic property. This reflectance property is called lustre. The most common types of lustre are:
Metallic: the look of metals. Opaque minerals are in this group, like native gold, native silver, native copper, pyrite, chalcopyrite, galena, pyrolusite and chassiterite.
Submetallic: a poor metallic lustre. Minerals, which are opaque but their reflecting is little light, are in this group, like native arsenic, graphite, liver ore, magnetite and ilmenite.
Adamantine: very gemmy crystals, like diamond, sphalerite, greenockite, cerussite, anglesite and zircone.
Vitreous: the most common lustre, it simply means the look of glass. Quartz, calcite, dolomite, malachite, barite, gypsum, lazulite, beryl, granates and feldspars are in this group.
Pearly: the look of a pearl. It is characteristic at the perfect cleavage minerals, like gypsum, mica, brucite and apophyllite.
Greasy and waxy: the look of grease or wax. There are some minerals in this group, like nepheline, oapl and uraninite.
Silky: the look of silk, similar to fibrous but more compact. Clay minerals include to this group.
Special colours and lustres
Iridescence – Iridescence is generally known as the property of certain surfaces which appear to change colour as the angle of view or the angle of illumination changes. It is common on the surface of opaque, metallic minerals.
Shiller – It caused by small, laminar crystal inclusions (hematite, mica or lepidocrocite) inside the minerals.
Asterism – Asterism is a well-known light effect in some gemstones. The effect is caused by minute acicular (needle-like) crystals of probably rutile or sometimes other minerals that are included in the host mineral. These minute crystals are microscopic, but there are thousands of them and their combined effect is to diffract light into these bands that appear as rays of light.
Cat’s eye – Cat's eyes are similar to asterisms and are caused by the same inclusions of minute crystals. But in this case the band on light is limited to one band that shimmers from the top to the bottom of the stone and appears like a glowing cat's eye.
Opalescence – Opalescence is a type of dichroism seen in highly dispersed systems with little opacity. The material appears yellowish-red in transmitted light and blue in the scattered light perpendicular to the transmitted light. The phenomenon is named after the appearance of opals. It is a characteristic fenomena of precious opal.
Labradorescence – It occurs in large crystal masses in anorthosite and shows a play of colors called labradorescence. The labradorescence is the result of light refracting within lamellar intergrowths resulting from phase exsolution (Pict. 1.7.).
Pict. 1.7. Labradorescence – labradorite
Adularescence – Adularescence is similar to labradorescence, produced most notably by moonstones. This effect is most typically produced by adularia (also known as precious moonstone), from which the name derives, but it appears in numerous other gemstones.
Hardness: is usually tested by seeing if some standard minerals are able to scratch others. A standard scale was developed by Friedrich Mohs in 1812. The standard minerals making up the Mohs scale of hardness are:
The Mohs scale of hardness
Hardness Mineral mode of
determination
1 talc easy to
scratch by nail
2 gypsum hard to
scratch by nail
3 calcite easy to
scratch by needle
4 fluorite easy to
scratch by knife
5 apatite hard to
scratch by knife
6 orthoclase scratchable by
rasp
7 quartz these scratch
glass
8 topaz
9 corundum
10 diamond
Cleavage and fracture: Because bonding is not of equal strength in all directions in most crystals, they will tend to break along crystallographic directions giving them a fracture property that reflects the underlying structure and is frequently diagnostic. The tendency for minerals to cleave or not and in which directions is very characteristic and therefore important to the identification of minerals. Cleavage is described in terms of how easy the cleavage is produced. From easiest to hardest to produce the terms are: perfect, imperfect, good, distinct, indistinct, and poor.
Fracture is a description of the way a mineral tends to break. Fracture occurs in all minerals even ones with cleavage, although a lot of cleavage directions can diminish the appearance of fracture surfaces. Different minerals will break in different ways and leave a surface that can be described in a recognizable way. The most common fracture type is conchoidal. Quartz has this fracture type. Unlike uneven, jagged has sharp points or edges that catch on a finger that's rubbed across the surface. Usually this indicates a metal such as copper , a metal alloy or some sulfides or oxides. Earthy is a fracture that produces a texture similar to broken children's clay. It is found in minerals that are generally massive and loosely consolidated such as limonite.