Diamond crystal from the Precambrian of Australia.

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5200 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals.

To find a native element in nature, it must be relatively non-reactive and there must be some concentration process. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state.

The element carbon occurs principally in its native state as graphite (C) and diamond (C). Graphite is the common & far less valuable polymorph of carbon. A scarce polymorph of carbon is diamond. The physical properties of diamond and graphite couldn’t be more different, considering they have the same chemistry. Diamond has a nonmetallic, adamantine luster, typically occurs in cubic or octahedral (double-pyramid) crystals, or subspherical to irregularly-shaped masses, and is extremely hard (H≡10). Diamonds can be almost any color, but are typically clearish, grayish, or yellowish. Many diamonds are noticeably fluorescent under black light (ultraviolet light), but the color and intensity of fluorescence varies. Some diamonds are phosphorescent - under certain conditions, they glow for a short interval on their own.

Very rarely, diamond is a rock-forming mineral (see diamondite - <a href="https://www.flickr.com/photos/jsjgeology/14618393527">www.flickr.com/photos/jsjgeology/14618393527</a>).

The Western Australian diamond shown above is a brown octahedral crystal from the 1.178 billion year-old Argyle Lamproite (a.k.a. Argyle AK1 Lamproite, Argyle Diatreme, Argyle Lamproite Diatreme, Argyle AK1 Pipe). This diamondiferous unit was discovered in October 1979 using the classic prospecting technique of identifying indicator minerals in stream sediments. It is a unique diamondiferous unit due to its relative abundance of pink diamonds. Elsewhere on Earth, pink diamonds are exceedingly rare (two famous examples of subtly pink diamonds were found at Golconda, India and Tanzania, Africa). Pink diamonds from the Argyle Lamproite can be intensely pink, unlike those from outside Australia.

Several different colors of diamonds are processed at the Argyle Mine, including clear (“white”), pink, brown, yellowish-brown, and a few unimpressive blue diamonds. About 5% of Argyle diamonds are gem-quality. The public has been tricked into buying ugly brown diamonds (see above example) by their being marketed as "cognac diamonds" or "chocolate diamonds". Light brown-colored diamonds are cunningly marketed as “champagne diamonds”.

The two major facies of host rocks for Argyle diamonds are olivine lamproite lapilli-ash metatuff and sandy lamproite tuff (see: <a href="https://www.flickr.com/photos/jsjgeology/14636757350">www.flickr.com/photos/jsjgeology/14636757350</a> and <a href="https://www.flickr.com/photos/jsjgeology/14843297313">www.flickr.com/photos/jsjgeology/14843297313</a>). The Argyle Lamproite Pipe erupted during the mid-Mesoproterozoic, at about 1.178 billion years ago. The included diamonds are much older (~1.6 billion years). The Argyle Pipe is fortuitously preserved. Nearby pipes have been eroded down to their roots. A quartzite ridge adjacent to the Argyle has protected it from erosion.

The pink coloration in some Argyle diamonds is apparently not the result of chemical impurity, which gives most diamonds their color variation. The pink color appears to be related to metamorphism-induced microfaulting or slippage of carbon atoms within the diamond crystals.

Geologic context: lamproite pipe emplaced along a splay of the Glenhill Fault, near-eastern margin of the Halls Creek Mobile Zone, eastern edge of the Kimberley Craton.

Location: Argyle Mine, near the headwaters of Smoke Creek, west of Lake Argyle, northwestern Western Australia (~16° 14’ South, ~128° 23’ East)

The Argyle write-up above is mostly synthesized from info. provided by Karen Rice (pers. comm.), Boxer et al. (1989), Jaques et al. (1989a, 1989b), and Pidgeon et al. (1989).

The latter four references are published in: