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Specimen: Opalised Belemnite
Age: Lower Cretaceous
Location Found: Lightning Ridge, New South Wales, Australia
Belemnites were marine animals belonging to the phylum Mollusca and the class Cephalopoda. Their closest living relatives are Squid and Cuttlefish. They had a squid-like body but, unlike modern squid, they had a hard-internal skeleton. In the animal’s tail, the skeleton formed a bullet-shaped feature sometimes referred to as a guard, but more correctly termed a rostrum. These are the parts that are normally found as fossils. Belemnites lived during the Jurassic and Cretaceous period, which lasted about 135 million years.
Opal forms in cavities within rocks. If the cavity is there because part of a living thing – for example a bone, shell or pinecone – was buried in the sand or clay before it turned to stone, then the opal can form a fossil replica of the object that was buried.
A fossil is simply “the remains or traces of an ancient animal or plant preserved in rock”. Opalised fossils form in ways similar to other fossils, except that here they are preserved in silica. Elsewhere, fossils are preserved in minerals such as agate, pyrite or limestone.
The sediments that buried plant and animal remains in the opal fields were rich in silica from ancient volcanoes, so here we have fossils preserved as silica in the form of opal.
Opalisation of plants and animal remains happens in two ways, and at Lightning Ridge, a combination of the following two processes is seen in many specimens.
Internal details not preserved (‘jelly mould’ fossils). Opal starts as silica dissolved in water. When the silica solution fills an empty cavity left by a shell or bone that has rotted away – like jelly poured in a mould – it may harden to form an opalised cast of the original object. In these fossils, outside features can be beautifully preserved, but the internal structures are not recorded.
Internal details preserved. If the silica seeps into the organic material before it decomposes, then the organic molecules can be replaced by silica. This preserves very fine details of structures inside the bone or plant. When the silica is transparent, this internal anatomy is visible from the outside: the fossil is ‘see through’.ACTUAL AS SEEN: The image shows the EXACT unique specimen you will receive. Measurements are as accurate as possible, though be aware measuring precise dimensions can be difficult from irregular shaped items. Whilst we portray colours as close to life-like as we can, colours will vary if taken in sunlight, indoors, from monitor to monitor and device to device.