Irish Green Marble - calcsilicate marble from the Ordovician of Ireland.

In the commercial decorative stone trade, “marble” is used to refer to a wide variety of relatively soft rocks (H = 3 to 5) that will take a fine polish. These include true marbles and rocks that aren’t marbles, such as limestones, tectonic breccias, and serpentinites.

The true marbles are calcitic, crystalline-textured metamorphic rocks. They form by intermediate- to high-grade metamorphism of limestones. They have been used for millennia as building and monument stones (e.g., ancient Greek & Roman archaeological sites and Michelangelo’s sculptures).

The attractive greenish-colored rock shown here is a marble rich in silicate minerals, principally serpentine. Metamorphic rocks dominated by both silicate and carbonate minerals are called "calcsilicates". Serpentine marbles usually form by metamorphism of dolostones. This calcsilicate is Ordovician in age, but the precursor (protolith - the original rock before metamorphism) was Neoproterozoic in age.

Stratigraphy: Connemara Marble Formation, upper Appin Group, Dalradian Supergroup, Neoproterozoic; metamorphism during the Ordovician, at ~466 to 478 Ma