Connemara marble

It has been used since Neolithic times and it is the national gemstone of Ireland.

Connemara marble, also called Irish green, is a rare green marble from the west of Ireland, found at places including Lissoughter in Recess, County Galway, and around Clifden. Its colour ties it so closely to Irish identity that it was named the national gemstone. It has been quarried and worked in Connemara since the eighteenth century, going into cladding, banisters, columns and church fittings across Ireland and Britain, and in the late nineteenth and early twentieth centuries it was exported to the United States in quantity for civic and educational buildings.

It is often mistaken for something it is not. Connemara marble strongly resembles verd antique, the green serpentinite breccia of the Mediterranean, and the difference matters: this is an actual marble rather than a breccia, despite its very high serpentine content.

The mineralogy explains the colour. Dolomite and calcite are the main components, with varying amounts of serpentine, diopside, forsterite, tremolite, clinochlore, phlogopite, omphacite and talc, and it is the serpentine that produces the characteristic green.

The rock began as mud. It was laid down as a limestone mud in the Neoproterozoic, and a team from Trinity College Dublin, the University of Galway, Connemara Marble Industries and the National Museum of Ireland describes what happened next: amphibolite-grade metamorphism of an impure siliceous dolomitic limestone during the Grampian orogeny, between 475 and 463 million years ago.

Then it was cooked twice. Olivine, diopside and tremolite formed alongside calcite and dolomite at the peak of that metamorphism, and a later pervasive hydrothermal event grew serpentine extensively over the olivine and diopside. The stone that results is a sillimanite-grade ophicarbonate with intricate corrugated layers running from white through sepia into every shade of green.

That same team has proposed it as a Global Heritage Stone Resource, which is the formal way of saying a stone matters to human culture and not only to geology.

The quarries are private and still working; the visitor centre and Galway shops sell verified Irish material.