This is my latest find - a 16 x 18 x 1 cm slice of Spherulitic Rhyolite - the same material and process that can form thunder-eggs. I got this from a Sydney Rocks area gift shop (how appropriate!).
I particularly like the variety of colours and shapes and see the little blue one that looks like Australia except Tasmania is on the wrong side?
This piece comes from the Mt Hay area near Wycarbah in Central Qld. The little town is famous for its Mt Hay Thunder Egg Park. There is a large rhyolitic lava flow at Mt Hay. The process is that gas bubbles were trapped in the lava flow during solidification, creating pockets which were later filled or partially filled by lava flows of siliceous material which may be agate, jasper, quartz or other materials. The rhyolite may be varying shades of green, brown and red, while the spherulites encased may be many colours including blue, red, brown and amethyst.
Note for Quilters: Some of Kaffe Fassett's fabrics have been inspired by geology!