Nature’s ready-made sculptures

These sandstone concretions of quartz and calcium, known as gogottes, were formed in mineral-rich waters during the Oligocene epoch — around 30 million years ago — when the global climate was cooling and deciduous forests replaced tropical ones.

Found in the forest of Fontainebleau, south of Paris, they are much prized for their sculptural appearance.

An extinct group of sea molluscs that were wiped out by the same event that probably killed off the dinosaurs, ammonites are an incredibly important ‘index fossil’, allowing geologists to determine the age of a section of Earth. Their closest surviving relatives are the nautilus and the cuttlefish.

Although most finds are only the size of a hand, larger examples, with their perfect spirals, make great sculptural pieces.

Minerals and meteorites often come to life when they are segmented to reveal their inner beauty. The translucency of the olivine and peridot crystals in pallasite meteorites, for example, is best revealed when sliced.

Agates, rose quartz, iron meteorites, fossil wood and tiger iron all have a crystalline structure that makes sliced examples perfect decorative collectables. Segments from larger specimens tend to have the advantage of being more accessibly priced, with estimates starting from around £1,000.

The fiery flashes of colour in opal make it a highly prized gemstone, but this iridescence can also be found in other fossils and minerals.

Ammolite, for example, found in Alberta, Canada, has been given gemstone status. Its dancing, metallic colours derive from layers of aragonite in the shells of ammonites that have been compressed over the course of 75 million years. The iridescent effect they create is similar to that caused by a film of oil on water.

Also from Canada, the mineral labradorite is named after its ‘geological type area’ — the term used to refer to the locality where a particular specimen was first discovered — and typically displays flashes of bright yellows, blues and greens.

The vast majority of fossil finds are fragmentary pieces of shell, bones and other hard body parts. Very occasionally, however, near-complete skulls — such as the example above — or entire skeletons are unearthed, providing us with a better picture of how the creature appeared when it roamed the Earth millions of years ago.

Most complete skeletons are found in marine deposits, where the creature was quickly covered by sediment and preserved as immense geological forces turned it to stone.

Meteorites represent the oldest category of object available for sale at Christie’s. Some have been dated at around 4.5 billion years old — which makes them a third as old as time itself.

Prized by collectors both for their appearance and their scientific interest, they are viewed by many as works of art from outer space.

Petrified fossils can be used just like any other building material — transformed into bathroom tiles, garden statuary, bowls, decorative objects and furniture.

When cut and polished, ‘rainbow’ petrified wood from northern Arizona, for example, can make a beautiful table top, with patterning that has been compared to the abstract paintings of Gerhard Richter.

Many of the top prehistoric predators shed teeth during their lives. With little to cause them to decompose, their calcified structures have survived well in the fossil record.

Larger, better-preserved specimens are more desirable, although their serrated edges can be extremely sharp, and have been known to cut the fingers of an unfortunate Christie’s cataloguer.

Tyrannosaurus rex  teeth are particularly highly prized, as are examples from the megalodon — among the largest prehistoric shark teeth known. This extinct giant of the seas could grow to about three times the size of a great white shark.

Returning from his second voyage to the North-West passage in 1577 the privateer Martin Forbisher (1535-94) carried as ballast for this three ships several hundred tons of ‘gold-bearing’ ore. After much heated debate at the assaying, involving the mathematician and alchemist John Dee (1527-1609), the true nature of the mineral was revealed as pyrite, or ‘fool’s gold’.

Such is the quality of the yellow lustre from pyrite that many have been drawn to it over the centuries.

The atomic arrangement of iron and sulphur atoms give pyrite its cubic crystal structure, and a few regions produce clusters of perfect cubes — a rare and wonderful example of straight lines occurring in nature.