Describing mineral colour |
The colour of minerals |
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Colour is often the first feature of a mineral that the amateur rock hound or collector notices. and one of the easiest to define.
Unlike in other sciences, where colour is often described quantitatively by reference to the measured wavelength or a colour chart, minerals are usually described in terms of common colour terms (e.g. red, blue, green, yellow, orange),or Intergrades between them (e.g. yellow-green, or yellowish green). It's also a simple matter to look up minerals in terms of their colour, because most inventories (e.g. Mindat, include the common colours in which they occur. Several resources can also be found online, listing minerals by colour - for example at Minerals.net. 
That said, descriptions of colour aren't always consistent or reliable. One reason is that different people seem to see colours in different ways (and many people have some degree of colour blindness in. certain spectra). The way we that perceive colours is also affected by context - e.g. by shadows and by the brightness of the light, and by the colour of adjacent objects. In addition, we may see different colours depending on which part of the mineral we look at and how well it's been prepared (see panel to right).
To define colour with any accuracy, therefore, it's important to standardise the conditions, Crucially, the mineral should be clear of dust or surface staining (e.g. by organic matter), dry, and viewable on different faces (to help recognise any natural variation in colour that may exist. it's also important to view any mineral under the best (and standard) conditions. Typically this means good but not direct light (direct light may cause reflection which may mask the 'true' colour). To download a list of common minerals by colour click here. |
The colour of any object is not an inherent property of the thing itself, but the result of the ways in which light is reflected or transmitted by the object. In the case of minerals, this depends on the chemistry of the mineral and the physical arrangement of the atoms of which it's formed. These vary between minerals, which is why they often seem to have different (and distinguishing) colours. However, variations in colour may also be seen between different crystals of the same mineral type, and even within an individual crystal (e.g. between one face and another). For all these reasons, great care is needed in trying to deduce the identity of a mineral from its colour. Often colour is better at telling you what a mineral isn't than what it is!
These colours may give a basis for distinguishing between minerals, but this isn't always the case. Many minerals show a range of colours, often depending on minor variations in their chemistry (for example, the presence and quantity of minor impurities) or their crystal structure. Colours can also vary within a mineral. Variations in chemistry may occur for example, because of differing conditions as the crystal grew or weathering after it was formed.. Strain, as a result of tiny movements in the rock, can create variations in colour due to distortion of the mineral's internal structure. Likewise, different faces of the crystal present different views of the atomic structure of the mineral. with the consequence that light reacts from them in different ways, producing different colours. The diagram below summarises the way light reacts with a mineral to create the colour we perceive. It's not the complete story, however, for light can also pass through the mineral and be emitted on a different face. When it does so, it emerges as light with somewhat different from the reflected light. Moreover, different rays of light pass through the mineral along different pathways and thus take different times to complete their journey. This produces varying colours when the emerging light is observed - something best seen with the aid of a thin section of the mineral and a microscope. |
