Boulder opal is precious opal that has a sandstone or ironstone base. Often, precious opal is too thin to allow it to be cut to the depth of the gemstone so the matrix rock is cut with it, making the material cheaper than pure precious opal. The paler stone has a layer of white precious opal and the darker slow has a layer of black opal. Stronger than precious opal because of the matrix, boulder opal is an easier stone to work with and suitable for use in a ring.