The 17-Million-Year-Old Lake That Boiled Australia's Only Zeolite Gem

On the Liverpool Plains of northern New South Wales, a 17-million-year-old volcanic lake did something no other lake in Australia has done: it turned volcanic ash into a gemstone.

The stone is erionite — a zeolite mineral so rare in gem quality that the deposit near the town of Werris Creek is the only known source in the country. Pale green needles, finely bundled into fibrous crystals, grew not in molten rock but in the quiet chemistry of an ancient lakebed.

The Lake That Chemically Boiled

When volcanoes erupted across eastern Australia during the Miocene, they left behind more than lava flows. In the Liverpool Plains region, eruptions formed depressions that filled with water, creating shallow alkaline lakes. The ash that settled into these lakes was not inert.

Alkaline lake water, rich in sodium and potassium, reacted with the volcanic glass over thousands of years. The glass dissolved and recrystallised into zeolite minerals — microporous aluminosilicates with a structure like a molecular sieve. Most of the world's zeolites form this way, but almost none grow into transparent, facetable crystals.

At Werris Creek, conditions were just right. The lake chemistry, temperature, and duration of alteration allowed erionite to form as distinct hexagonal prisms rather than the usual powdery masses. The crystals are tiny — rarely more than a few millimetres long — but they are clear enough to cut.

A Gem That Looks Like Frozen Sunlight

Erionite belongs to the zeolite family, a group of minerals that typically form in cavities of volcanic rock or in altered ash beds. Most zeolites are soft, brittle, and dull. Gem-quality erionite is an exception.

The Werris Creek erionite ranges from pale green to golden yellow, with a vitreous lustre that catches light like a sliver of bottle glass. Under magnification, the crystals show a fibrous internal structure — parallel bundles of needle-like growth that give the stone a subtle chatoyancy, or cat's-eye effect, when cut as a cabochon.

Only a handful of other places on Earth produce gem erionite: the Puna Plateau of Argentina, the Nandan area of China, and the Bear Creek矿床 in Oregon. The Australian deposit is the smallest and least known, but the crystals are among the finest.

The Zeolite That Breathes

The name "zeolite" comes from the Greek zeo (to boil) and lithos (stone), because when heated, zeolites release their trapped water as steam. Erionite does this too, but it also does something more remarkable.

Its crystal structure is a network of channels and cages, each about the width of a small molecule. These cavities can trap gases, ions, and even water molecules, then release them under the right conditions. Industrial zeolites are used in water purification, nuclear waste treatment, and as catalysts in petroleum refining. Natural gem erionite is too rare for such uses, but the same chemistry that makes it a gem also makes it a sponge.

The same crystal lattice that holds a gem's colour can also hold ammonia, methane, and carbon dioxide — a stone that remembers the lake that made it.

A Vanishing Deposit

The Werris Creek erionite occurs in a thin layer of altered tuff — volcanic ash that settled into a Miocene lake and never left. The bed is exposed in only a few small outcrops on private farmland, and the crystals are scattered sparsely through the rock.

Collectors have worked the deposit for decades, but the supply is limited. Unlike sapphire or opal, which occur in tonnes of gravel, gem erionite is measured in grams. A single productive pocket might yield a dozen facetable crystals.

The lake that made them dried up long ago. The volcanoes that filled it are eroded to stumps. Only the zeolite remains — pale green needles frozen in ash, waiting for someone to notice they are beautiful.