Weighing 14 metric tonnes and standing at 5 metres tall and 2 and a half metres wide, this vividly coloured geode is the largest amethyst ever to be offered at auction. Originating from Uruguay, the amethyst sparkles in various hues of lilac, rose and blue depending on the light shined upon it. To add to the colour, there is the presence of very rare yellow hemispherical calcite crystals. It comes to Bonhams Natural History Sale in Los Angeles on December 3, and has an estimate of $300,000-400,000.

Tom Lindgren, Co-Director of the Natural History Department at Bonhams said: 'For me an item of particular note is the spectacular fossil of an Eocene turtle. Superb preservation and beautiful natural coloration make this magnificent very large turtle specimen, from the famed Green River Formation in Wyoming, particularly special. It's awe-inspiring size is matched only by its fine detail and astounding completeness.'

The Eocene Turtle, Axestemys byssinus, existed around 100 million years ago and survived the K-T extinction event which caused the end of dinosaurs and most other terrestrial species on earth. Framed in tropical walnut and fitted with a cleat system for ease of display on any wall of conventional construction. It is estimated at $200,000-300,000.

Co-Director of the Natural History Department at Bonhams, Claudia Florian, said: 'We are very excited to be offering so many fascinating items in our Natural History Sale. In all 5 sections, Dinosauria, Fossils, Meteorites, Minerals and Décor, there are rare and remarkable pieces from the Earth's history before humankind. With a sale range of $500 to 300,000 this sale is suited to every bidder, from the seasoned collector to the enthusiast seeking to start a collection.'

Other Highlights Include:

• A Gem-Encrusted Malachite and Mother-Of-Pearl Chess Set by Konstatin Libman. The board consists of intarsia squares of malachite and mother-of-pearl. The chess pieces carved of malachite and white agate and are mounted with hand-fabricated silver. The pieces are accented with emeralds and peridots, as well as black and white cultured pearls. Estimated at $25,000-30,000.

• A research quality cast of a Tyrannosaur Skeleton. At 23 foot long and over 10 foot tall the Tryannasauras Bataar was the Apex predator in Asia while its cousin, The T-Rex, roamed Africa. This fully mounted tyrannosaur skeleton was meticulously produced of cast bones taken from a mould of the original specimen. It is estimated at $35,000-50,000.

• Artistically Rendered Megalodon Jaw with Cast Teeth. The megalodon is the largest shark, as well as the largest fish, that ever lived. Its name, meaning giant tooth is more than apt, as the huge shark's teeth are almost three times the size of the contemporary great white sharks. Offered is a museum-display-ready representation of Megalodon jaws with teeth cast from original Megalodon teeth. It is estimated at $7,500-10,000 and will be sold without a reserve.

• Muonionalusta Iron Meteorite discovered in Norrbotten, Sweden in 1906, Muonionalusta formed when two large asteroids were destroyed by collision in the asteroid belt billions of years ago. Muonionalusta is classified as a fine octahedrite and has one of the most beautiful Widmanstatten patterns when it is etched. Widmanstatten patterns form when the two alloys of nickel, kamacite and taenite, separate as the iron core cools at a rate of only a few degrees per million years. These patterns are not found in any materials on Earth. It is estimated at $25,000 to 30,000.