Mookaite is an unusual material. It is mined in Western Australia from the Windalia Radiolarite. A "radiolarite" is sedimentary rock that is made up of radiolarian tests. Radiolarians are tiny organisms that drift in the upper waters of the ocean and produce a thin shell called a "test." When the radiolarians die their tests sink to the bottom. That is how the Windalia Radiolarite was deposited. After deposition of the tests, groundwater rich in dissolved silica flowed through the sediment and precipitated solid silica in the pore spaces. Where this process completely silicified the rock is where the gemstone material mookaite was formed.
Size about that of a 25 cent piece.