Fossils Information & FAQs
Fossils On this page you will find background information about fossils.

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 fossils so will be regularly adding more ...

... do come back and see !
The Natural History Museum - Fossils

To find out more about fossils


to go the the fossils section of the Natural History Museum's site
Rocks and Minerals Information Page

Charlotte Bailey Rocks Fossils and Minerals


to go to my Rocks and Minerals Information Page
Q: What is meant by "Natural Formations" ?

This means that the item has been found in the form it is, quite naturally ( See stock pic of double ammonite and shell formations) It is quite common for collectors to create an artistic arrangement of fossils often to great effect but obviously a man made formation is not the same as finding a naturally occurring one. Hence the perceived value is much higher to carefully extract an original collection, in its entirety.

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Q: How are fossils formed ?

The most common way is for the original shell or bone to be buried in sediment, as even the hardest parts will be broken down if they are exposed to the weather, bacterial action and of course scavengers. The faster the creature or plant is buried after death the greater the chances of preservation. When filled with sediment and prevailing minerals the external imprint is maintained and over time turn into completely different material. Often the ingress of minerals will provide an additional beauty such as the lustre of Pyrite, Opaline and Silica. Peat bogs and Tar pits may preserve whole vertebrae.

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Q: How does wood turn to stone ?

When a log or branch from a tree falls, if it is quickly submersed in mud, before the decay that would happen should it be exposed to air and bacteria take hold, then the mineral-rich ground water soaks into the wood or log. This ground water then infiltrates the spaces between the cells of the wood. As the wood breaks down a weak acid is generated and this enables the minerals to move out of the ground water solution and fill the spaces with crystals. This process is called permineralization or petrification, it can occur in a very short space of time geologically speaking, often less than a hundred years but can take much longer and the wood disintegrates further over several thousand years.

The next process occurs as the minerals replace the original wood tissues sometimes as slowly as molecule by molecule. Some parts of the wood, as in the case of the layers of annual rings that grew in the springtime, are the first and most vulnerable to decay and thus become mineralised earlier than the harder portions. This results in the preservation of the distinctive ring patterns you see in a freshly cut tree stump.

The final composition of the petrified wood will then usually become some form of Quartz or Calcite. It will depend upon what minerals are in the ground water. For example a silica solution may often cause a vivid coloured Agate, Jasper or even Opaline species (All from the family of Quartz) The different colours are the results of trace minerals in the solution with Copper, Cobalt and Chromium giving green or blue, Iron giving red (pinkish if not too strong a component),brown or yellow and carbon inevitably black. Occasionally both Agate and Calcite indicate that both Calcium Carbonate and Silica were in the water although not necessarily at the same time. When the wood rots in mud before petrification it can leave a mould behind which if it is filled with crystalline minerals can create a cast and this may preserve all the wonderful bark detail.

Wonderful examples can be seen in the State Petrified Forest in Arizona USA and White Cliffs in Australia.

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Q: What is Fossil Lake or The Green River Formation ?

The Fossil Lake dates to the early Eocene period over 50 million years ago. The water drained from the recently arisen Rocky Mountains created an amazing inter-mountain basin which came to be called Fossil Lake. The climatic conditions in this area were sub-tropical and pretty much the same as that of Florida to-day. This special environment lasted for around 2million years and was home to Palm Trees, Turtles, Crocodiles and an abundance of fish now extinct. It would seem that a variety of unique conditions prevailed to encapsulate some of the best preserved fossils, especially fish, ever discovered.

The rich abundance of fossils in the sediments of the Lake were first discovered in the 1850's close to the town of Green River in Wyoming, hence the name now known as The Green River Formation. Unlike other fossilised forms in the rest of the world, where only fragments of fossilised fish are found, this area is home to some of the most perfectly preserved whole fish species.

Experts in Palaeontology and Sedimentology have theorised that the lake was sufficiently deep that it may have been "Anoxid" or devoid of Oxygen, this would have explained the superb fossilisation as the fish would not have been disturbed by scavengers and this would have been instrumental in preventing the further decomposition of the specimens. As the sedimentary layers contain such an abundance of fish it has been mooted by the scientists that it may have been as a result of a series of events such as Algal blooms that so many fish were killed on such a large scale, as were other plant and animal life which died and sank to the bottom, as in lakes and ponds to this day.

Storms, and rainfall in general, brought down from the Rocky Mountains, a source of mineral rich-water which would ensure their preservation.

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Q: What makes the prices of Ammonites (and all items) vary so widely ?

There are several factors and many apply to all the rocks fossils and minerals. Rarity is an obvious differential but relative quality and condition are very important factors and the aspect of what makes something of "better" quality will vary in each particular subject. In most cases it is a matter simply of appearance but often a particularly special characteristic.

As far as Ammonites are concerned they will vary in quality for several reasons... for example the ones that are from Morocco such as those displayed on the website are extremely well preserved this is due to the climatic differences as the much dryer conditions mean that unlike a similar specimen possibly from the UK they have not suffered the same weathering from acid rain and erosion. There are also many different types of ammonite which will have different characteristics depending on where they came from and the prevailing mineral deposits surrounding the fossilisation.

For example the Craspedites from Russia which are distinctive black and white due to the black calcite which divides the chambers (See stock pic). The slimmer varieties known as Clioniceras, the ridge backed Perisinctus and so on. The detail in fossilisation that has been retained is very important for example the Trilobite will attract a higher price depending on the detail maintained some examples just illustrate an outline and none of the detail of better and rare specimens (See the stock pic for a superb Trilobite head for example).
Whole fish are rare throughout the world as high levels of oxygen in the water will effect the rapid breakdown and decomposition of the fish bones, with the exception of The Green River formation where the three basins of the Colorado river formed an exceptional environment with heavy sediment allowing a wonderful amount of now extinct fish to have been preserved (See fish plaque stock for Knightia and Priscacara)

Fair trade and properly sourced stock is also an indication of price difference, as many specimens are from the some of the poorest regions of the world. The prime finds are usually obtained by reputable companies and the higher prices paid.

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Q: Why are fossils often coloured and what causes this colouration ?

The formation of a fossil is often referred to as petrifaction (as in 'petrified' literally meaning turned to stone). This generally involves permineralisation and/or replacement. Permineralisation (which includes pyritisation or silicification) is where water containing traces of minerals works its way into the voids and spaces in the remains of an organism and then over time those minerals precipitate out forming solid deposits of that mineral on the original form, like a cast. Replacement is where mineral bearing water actually replaces the cells and structure of that original organism. The latter, particularly if it happened very slowly, lead to the most detailed fossils.

The minerals ingressing during petrifaction will determine the colour of the fossil Below are just a few examples. If there is ingress of more than one mineral then the colours may change not least because of different minerals reacting with each other. This, therefore, is just a rough indication only.

Red to pink: Iron Ferric, Manganese

Orange: Copper

Yellow to Gold: Iron Ferric, Uranium, Pyrite

Green: Iron Ferric, copper, Cobalt, Chromium , Uranium, Nickel

Blue: Copper, Manganese, Cobalt, Iron Ferric

Black to Silver-Grey: Iron Ferric, Uranium

Brown: Iron Ferric

White to Grey: Silicon Dioxide

For further information you can see the FAQ on identification of minerals by colour here .

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Q: What is amber ?

Amber occurs as a defence mechanism on certain types of trees, usually ancient coniferous trees. If the tree bark was punctured, or damaged by disease a sticky resin (as opposed to sap) would ooze from the area and seal and sterilise it. This could also occur if a branch had broken off. Aroma from the resin is believed to both attract certain types of insects, or repel others.

Some of the oldest amber found dates back to approximately 320 million years ago but it became more abundant around 150 million years ago when it began to be found to have inclusions of insects and other plant matter. Indeed it takes many thousands and millions of years to form into the amber as we know it.

It can be found in quantity in the Baltic area and the Domincan Republic. It is also found in Columbia where this is known as Copal Amber which is tens of thousands of years old rather than millions.

It has been used for ornamental use since the Stone Age and its' classical name was Electron which was connected to a term meaning "beaming sun". The colour ranges from very pale yellow through gold to dark brown. It is translucent giving us the perfect opportunity to spot any insects and plant life trapped in it.

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