The above opal was discovered in the opal fields of Coober Pedy by opal miner John Dunstan in 2003. It’s actually an opalised fossil, from an extinct cephalopod called belemnitida that existed during the Mesozoic era. It is worth over $1 million and will soon be displayed at the South Australia Museum. (Source)
The usual remnant of these cuttlefish like cephalopods are their shelly shields, usually cone shaped, like the opalised ones I shared in a previous post (see http://on.fb.me/1TFM2iH). This example of Phragmoteuthis conocauda from the collection of the Munich Paleontological Museum illustrates well what I said in that post about their tentacles and soft parts.
While I did cover this lovely gem material before (see http://on.fb.me/1H2oeQy), the advent of #cephalopodweek gave me an excuse to trot out these lovely photos I’d accumulated. It is often mislabelled as opal, or opalised ammonite, though gemmy silica with play of colour has nothing to do with it, and the sheen comes from a totally different mechanism. Ammolite is in fact fossilised mother of pearl, like you see in a mussel or nautilus shell (their remaining closest living relatives), and gives us an idea of how these creatures looked as they swam thorough the seas of the Mesozoic.
Opals play of colour is due to regularly stacked and spaced silica spheres diffracting the wavelengths of light, while ammolite’s shifting red and green glows are due to thin film interference between rays of light reflecting from different layers in the stacked strata of aragonite crystals that form the nacreous shell. The same phenomenon it responsible for the sheen of oil or petrol on a puddle of water. Thicker layers of nacre produce the red to green hues and thinner ones blues to violet, reflecting which wavelengths of light they approximate to.
Opalised fossils form in the same way as most fossils, by a process of fossilization called permineralization, AKA petrification. After an organism’s soft tissues decay in sediment, the hard parts are left behind. Water seeps into the remains, and minerals dissolved in the water seep into the spaces within the remains, where they form crystals. These crystallized minerals cause the remains to harden along with the encasing sedimentary rock.
Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight, but is usually between 6 and 10%.
When opals are mined they can change significantly as their water evaporates over a time period of about a year until they stabilize.
Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. The internal structure of opal makes it diffract light, and it can vary in optical density from opaque to semitransparent.
The specimen above is a variety of opal known as Opal-CT, or “lussatite”. Opal-CT consists of packed microscopic (150-300 nm) spheres made up of tiny microcrystalline blades of Cristobalite and/or Tridymite, with water content as high as 10% its weight.
This natural opalised Belemnite Fossil was kept by the miner and worn around his neck for 20 years. It was found in the dirt wall already in three pieces and set in silver to keep it together. If you’ve got a cool $16,000 to spare you could be the new owner of this amazing fossil.