Sometimes within old lava fields are found rounded “geodes” of silica often with a rhyolite exterior. These are known as agates, or Thunder eggs. Much like human beings, these geodes are built on the same basic pattern but each one is different from all the others. The silica is in the form of tiny microscopic crystals of quartz in a fibre or overlapping plate like configuration, known as chalcedony , agate and jasper being examples of this form of silica.