(53) Earth Science

What scientists found trapped in a diamond: a type of ice not known on Earth

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Most people are most interested in the colour of a mineral. This is especially important for choosing minerals as gemstones for jewellery. After all, jewellery has to match the outfit (maybe that’s why ‘‘diamonds are a girl’s best friend,’’ they go with everything)!

One of the problems geologists find in using colour to identify minerals of a certain group is that some minerals can be very different. Some colours are called idiochromatic. Their chemistry gives them their colour. Malachite which has a lot of copper is always green because copper gives it that colour.

Minerals that are usually colourless and take on the colour of small impurities are called pseudochromatic. Depending on the impurity, they can have a variety of colours. If a mineral contains bits of iron, it will take on a reddish colour.

Allochromatic minerals are generally colourless and transparent. They get their colour from the small changes in their crystalline make up or from structural flaws. In corundum, for example, the substitution of iron and titanium for aluminium gives a blue sapphire, while iron by itself produces a yellow sapphire.

Minerals like quartz come in lots of colours. Some of the colours that quartz can take are listed below:

* Colorless quartz,

* Rose quartz (all shades of pink),

* Milky quartz (white and whitish grey),

* Citrine quartz (yellow, yellowish brown, and orange),

* Smoky quartz (brown, brownish black, and black), and

* Amethyst (light to deep purple).

Many minerals, depending on their chemical content and formation, are found in different colours. Ruby and sapphire are both varieties of corundum with the same chemical composition (AlO3) and hardness (9), but two very different colours. Most people know that rubies are red and sapphires are blue.

However, just to keep you guessing, sapphires can also be colourless, green, yellow, or purple! Tourmaline is thought to have the greatest number of colour variations. A ‘‘chameleon of colour,’’ tourmaline is a prismatic crystal and is found to occur in seven different forms. These include elbaite (multicoloured), school (black), buergerite and dravite (brown), rubellite (pink), chromdravite (green), and uvite (black, brown, and yellowish green). Long, tourmaline crystals can be pink on one end and green on the other! They look like some rare cosmic gem from a science fiction movie.

A few minerals, like ruby, are fluorescent. They absorb blue and ultraviolet light and then release some of the energy back in the red part of the light spectrum.


The streak of a mineral is simple to remember. It is just what it says, a powdery streak made when rubbing a sample across an unglazed surface. The streak is a more dependable way to test a mineral than colour since it is nearly always the same for different minerals. Sometimes a hard sample must have a small bit crushed with a geological hammer to get a sample to test. A streak may be colourless, white, golden yellow, yellow, reddish brown, red, grey, brown, or black. The streak of a mineral is often not the same colour as the mineral appears to the eye. For example, the mineral crocoite is orange-red and its streak is yellow. Wulfenite can be orange, yellow, brown, grey, or greenish brown, but its streak is white.


Luster is the word geologists use to describe the way light reflects off the surface of a mineral or crystal. The amount of light absorbed and a mineral’s texture affect luster. The different types of luster consist of dull, metallic, vitreous (glassy), adamantine, pearly, greasy, silky, and waxy. These are pretty straightforward and were used by some of the earliest people in describing different minerals. For example, gold and platinum have metallic lusters, but not microcline, which has a vitreous or pearly luster. Most silicates, sulfates, halides, oxides, hydroxides, carbonates, and phosphates have a vitreous luster.

A diamond’s high luster or that of highly reflective, transparent, or translucent mineral is known as an adamantine luster. Zircon, cuprite, and some forms of sulfur and cinnabar have this type of luster. One thing to remember is that depending on the mineral and the environment in which it was formed, luster can be different in different parts of the same sample, as well as in different samples (from different places) of the same mineral.


Depending on the way minerals are bonded, the light will pass through a mineral in different amounts. When you can see right through a mineral like glass, it is said to be transparent. If the light is slightly blocked, making the mineral look foggy and unclear, it is said to be translucent. If a mineral sample is solid and lets no light pass through at all, it is called opaque.

A transparent mineral can be seen through, while a translucent mineral is hazy, and an opaque mineral lets no light pass through at all.

A sample’s transparency isn’t always the same all the way through. Crystals are often transparent to translucent across a sample. For example, amethyst and olivine crystals are usually transparent to translucent within the same sample. Opal is transparent to opaque across a sample, while copper and jamesonite are opaque.


When geologists are trying to figure out the identity of an unknown sample, they use the above-mentioned characteristics as well as specific gravity (SG). The density of a sample is measured in terms of its specific gravity.

Specific gravity is the ratio of the mass of a substance compared to the mass of an equal volume of water at a specific temperature.

To find the specific gravity of a mineral, compare its weight to the weight of an equal volume of water. For example, a specific gravity of 4 tells geologists that an unknown sample is four times heavier than water. Size doesn’t matter. A larger sample can have a lower specific gravity. This is the case with talc and mercury. A large amount of talc would have a lower specific gravity than a small amount of mercury. The specific gravity of talc is 2.8, while mercury’s specific gravity is 13.6. 

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