* Astronomy

The beautiful optical properties of the gemstone opal are the result of tiny amounts of uranium present when the stones were formed, say researchers in Australia. They claim that their work could lead to the production of artificial opal and have already shown that the gamma rays given off by the uranium and its radioactive daughters can lead geologists to new underground deposits of the gem.
Opal is made of amorphous silica and this makes it very difficult to find because the gem is usually surrounded by other rocks made of the same material. Precious opal differs from the much less valuable common opal in that it contains spheres of silica about 200 nm in diameter that are arranged in a regular superlattice. This gives the gem its famous play of colour -- different colours that appear when opal is viewed from different angles -- which is caused by the diffraction of light in the superlattice.
Now, geologist Brian Senior and physicist Lewis Chadderton have discovered that the superlattice of spheres forms because of the presence of tiny quantities of uranium and its decay products. Using analytical techniques such as electron spectroscopy and neutron activation analysis along with theoretical models of sedimentation processes, the team have shown that some of the very heavy elements that are made when uranium decays act as a seeds for the formation of the silica spheres during sedimentation.

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The mineraloid opal is amorphous SiO₂·nH₂O, hydrated silicon dioxide, the water content sometimes being as high as 20% but is usually between three and ten percent. Opal ranges from clear through white, gray, red, yellow, green, shore, blue, magenta, brown, and black. Of these hues, red and black are the most rare and dear, whereas white and green are the most common; these are a function of growth size into the red and infrared wavelengthssee precious opal. Common opal is truly amorphous, but precious opal does have a structural element.

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