kai

Lovely geode

These balls of quartzy rock record the shape of bubbles in long frozen basaltic lava. They were deposited after the flow had cooled by silica rich waters, possibly driven in a convection cell by the heat of cooling lava. What I love about them is the element of surprise; no one knows what marvellous landscapes hide within until the diamond tipped saw is sent a whirring. In the lovely example in the photo (sorry no scale available) layers of agate started to fill the ex bubble from the outside in. A layer of jelly like colloidal silica probably precipitated to create the horizontal banding, while crystals of lovely pink chalcedony (coloured by traces of iron oxide or manganese) and drusy points of water clear quartz grew inwards into the cavity.

Loz

Image credit: Captain Tenneal