Both precious and common opal is found in three major forms, or shapes;
- Nobby opal is a type of opal that is found as rounded nodules from the size of a pea to as large as a potato. These are created as a theory where clay balls under ground leaving cavities. Over time these cavities are filled in with silica which crystalise under temperature and pressure. Nobby opal is rounded or a witches cone shaped hat.
- Seam opal is usually found as a horizontal layer, or seam, in the opal bearing rocks. Found in cracks in the ground, creating slabs of opal, these are flat which is why it is called in seam, normally in thick sedimentary pieces.
- Fossil pseudomorphs (false forms) are plant, shell, bone or teeth fossils which have been replaced by opal.
SEAM- FLAT SHAPE OPAL