THE FIREWORKS INSIDE AN OPAL

Forget waiting for the Fourth of July.

- JIM BRACE-THOMPSON

Opal replacement fossils from Australia.

Twist a precious opal any day of the year and you'll unleash fireworks in your hand. What causes the brilliant explosions of so many colors in these wonderful gemstones?

WHAT IS OPAL & HOW DOES IT FORM?

At its heart, opal is just like quartz. Both are made of silicon dioxide (tiny bits of glass). Opal has a special twist. It holds water!

Because of that water, it can’t grow into perfect crystals like quartz does. Instead, rain or groundwater picks up lots of tiny silica grains as it moves through rocks. When this silica-rich water cools, it turns into a jelly-like gel that oozes into holes or gaps and slowly dries and hardens into opal.

Those cavities can be in soft sandstone or in bubbles left when lava cooled into basalt. In Australia, opal has even filled in spaces where ancient sea reptiles’ bones or clam and belemnite shells once were, making gorgeous fossils.

Above, left: Synthetic Gilson opal with large color flakes. Above, right: Boulder opals from Australia Below, right: Milky white common opal

BEWARE THE BLAH!

Not all opal is brilliant. Most of it is common opal, also called potch, and is rather blah. It is opaque and milky white, although impurities can stain it most any color. It does have a pearly sheen referred to as opalescence.

Ethiopian opal cut-and-polished

FUN FACTS!

Opal is the October birthstone.