Rocks and Minerals Information & FAQs
Rocks and Minerals On this page you will find background information about rocks and minerals.

Over time I have put together information, in the form of personal notes, not least in response to some of the questions that customers have asked me. From these I am now collating information sheets and frequently asked questions which I will be putting up on this page.

I never stop learning about rocks and minerals so will be regularly adding more ...

... do come back and see !
Natural History Museum - Rocks and Minerals

To find out more about rocks and minerals


to go the the Natural History Museum's minerals gallery
Fossils Information Page

Charlotte Bailey Rocks Fossils and Minerals


to go to my Fossils Information Page
Q: What is meant by "inclusions" ?

This literally means that the piece has acquired an additional feature in its formation, for example some quartz will clearly show such inclusions of minerals, Black inclusions tend to be Tourmaline rods, Green Azurite and Gold/Rust shades Iron trying to infuse. They make for very interesting specimens.

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Q: What is meant by Hydro-Thermal Veins ?

Many good specimens are obtained from mineral veins. High temperature fluids deposit minerals in cracks and fissures in rocks these are often called hydrothermal veins and are frequently worked as sources of ore, they often contain colourful specimens and good crystals. Sometimes weathered- out cavity linings called Geodes are lined with well-shaped crystals and many fine specimens of Amethyst occur in these. Many crystal formations occur in Hydro- thermal veins. The largest are not always the most spectacular and some of the smallest crystals are the most perfectly formed ones.

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Q: What is double terminated Quartz ?

Most crystals have a single pointed end but in the case of Double terminated this simply means they have a point at both ends. See stock pictures "Double terminated Quartz" one of my exhibition pieces is exceptional as it features both a double point and has an amazing inclusion of trapped water !

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Q: What is the difference between minerals and rocks ?

Minerals are naturally occurring chemical compounds with a known crystal structure. A particular mineral is thus defined not just as a chemical compound but also by its crystal structure. A mineral is also only one substance in a single consistent piece.

Rocks are made up of a mixture of things including minerals. Experts, if provided with pieces of sufficient size, can identify, usually under magnification, some of these different mineral components. For example, in Granite, which is a rock, one would usually see a mixture including one or two feldspars, some Mica, as well as Quartz etc.

The Earth's crust consists of rock which includes deposits of minerals.

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Q: What are the different types of rock ?

Sedimentary: this is made up of grains of sediment that have generally weathered from pre-existing rocks or minerals but can also include deposits of decayed life forms. It may be layered and may also contain fossils. Generally soft and may break or crumble easily.

Igneous: often formed by volcanic activity, these rocks are made up of interlocking crystals that form when molten rock cools. They rarely contain fossils but may contain gas bubbles.

Metamorphic: these rocks form when a pre-existing rock is subject to heat or pressure or a combination of both. The rocks that result often also have crystals formed by minerals growing over time on their surface.

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Q: Can one identify a mineral by its colour and what elements influence that ?

Great care should be taken in using colour as a positive identification of a mineral if one is not an expert in this field. Many different minerals have similar colouration. Formally a mineral is defined by its chemical and crystal structure, but, yes, you can use colour to help identify a particular mineral and its constituent elements.

Here are the types of colouration, and below examples of the elements that influence the colour of particular minerals.

Idiochromatic: this term relates to minerals that are self-coloured and remain unchanging and are predictable e.g. blue of Azurite, green of Malachite, red of Cinnabar, yellow of Sulphur.

Allochromatic: these have varied colours due to combination or impurities in their make-up, therefore depending on the other compounds, they have a changing or unpredictable variation in quality and depth of their shades e.g. Diamond, rose in Rose Quartz, yellow in Heliodor and Citrine, blue/turquoise in Amazonite and Beryl.

Pseudochromatic: colouration displayed will vary with different light diffraction and is therefore variable, but nevertheless a unique feature of the mineral. Good examples of this are shown in the precious Opal and also Labradorite. Labradorite, often known as Spectoralite, shows a wonderful "Iridescence" a term used to describe the type of colours one can see when blowing soap bubbles! ( Sir Isaac Newton 1642-1727 was the scientist who worked out that white light can be separated out into seven different colours)

N.B. Some minerals, such as Agate (which in its natural form is usually banded shades of blue, white, grey, brown, red and black), are receptive to artificial colouring and many ornamental pieces can be seen in vivid shades of pink, purple, turquoise, etc. none of which are natural.

Here now is a ROUGH guide to some of the major elements that influence colouration.

Chromium:- Cracoite(orange) - Phoeicrocoite and Ruby (red) - Uvarovite, Emerald, Zepeite, Grossularite, Jade, and Tourmaline (greens) - Alexandrine (green-red)

Cobalt:- Lusakite (blue) - Roselite (pink)

Copper:- Azurite (blue) - Malachite (green) - Cuprite (red)

Iron:- Lazulite (blue) - Almondite (red) - Cacoenite, Goethite, Citrine and Idocrase (yellow) - Aquamarine and Tourmaline (green) - Jade (green, yellow, brown)

Manganese:- Rhodacrosite (pink to red) - Spessarite (orange) - Ganophyllite (yellow) - Tourmaline (pink to violet) - Andalusite (green-yellow)

Nickel:- Annabergite (bright green) - Bunsanite (green)

Uranium:- Zepeite and Carnotite (yellow) - Curite (orange)

Vanadium:- Apophyllite (green) - Alexandrite (green-red)

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Q: What is a meteorite ?

Meteorites are pieces of rock and metal (often nickel and iron) that have broken away from much bigger extra-terrestrial bodies or asteroids and fall to Earth. This can happen when asteroids collide out in space. They vary immensely in size and shape-from small stone- sized pieces to the most huge formations.

Asteroids number thousands in the solar system and are believed to have been formed billions of years ago. Basically an asteroid is in outer space and a Meteorite is found on the ground but have the same properties.

Meteorites are rarely seen actually entering the Earth's atmosphere and hitting the ground, but are found all over the world, although many are lost in the oceans. They are not easily identified on the ground unless they land in deserts (as several have done in both Australasia and the USA) because they are much more obviously not part of the natural landscape - likewise if they fall in the Arctic or Antarctic.

For the cosmologists and cosmochemists their chemical structure provides some insight into the history of the solar system.

Most meteorites are rock or metal but Pallasite, one of the stony-iron meteorites, is particularly special and beautiful. It consists of nickel and iron within which is embedded a lovely olive green-gold mineral called, appropriately, Olivine which is a magnesium-silicate. Olivine in its purest form is what we call Peridot a semi-precious stone used in jewellery. You can see pictures of Pallasite in the my blog .

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Q: What are geodes ?

Geodes are unassuming rocks on the outside usually rounded or oval shaped and with an almost cauliflower-like external pattern. Inside however they have amazing crystal formations. Geodes are a thing of natural beauty and wonder.

To begin with, it is believed, that they began as a hollow bubble of gas or air inside volcanic or sedimentary rock. These cavities may also have been created by the decomposition of sea creatures under water leaving a hollow or even of ancient tree roots or animal burrows. Over tens of thousands of years up to millions of years with the right combination of water, minerals heat and pressure, crystals form inside. The crystals may "grow" and become large single crystals or tightly packed micro-crystals. When the cavities become completely filled they are referred to as nodules.

Amethyst geodes are amongst the most frequently seen with their beautiful dark purple to lilac crystals. The colours of all Geodes vary due to the combination of minerals in the water ingress to the rock.

Geodes in my stock list include whole Condor agates split in half and polished, Amethyst Geodes of all shapes and sizes, Celestine and the wonderful Rhyolite Geode rock book ends from Dugway in Utah.

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Q: What is a crystal ? OR What are crystals ?

A crystal is a solid body (generally a uniform mass of a naturally occurring mineral) with a geometrically similar shape.

This is caused because its atoms form a regular ordered, or structured, pattern which is repeated. So a large or small specimen of the same type of crystal will have exactly the same type of internal structure which can be recognised under a microscope.

Chemical conditions, temperature, pressure and environment will affect their growth. Some crystallise from watery solutions and some from molten rock during volcanic eruptions when lava swiftly cools. When magma cools sufficiently to condense into a liquid the resulting mineral-rich solution can seep into surrounding sedimentary rock, creating large crystals and Geodes such as in Amethyst.

There are seven three dimensional patterns: Cubic, Tetragonal, HexagonaL, Trigonal, orthorhombic, Monoclinic and Triclinic.

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