Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na2(Fe2+3Fe3+2)Si8O22(OH)2.

Riebeckite was first described in 1888 for an occurrence on Socotra Island, Aden Governorate, Yemen, and named for German explorer Emil Riebeck (1853–1885).

It occurs in association with aegirine, nepheline, albite, arfvedsonite in igneous rocks; with tremolite, ferro-actinolite in metamorphic rocks; and with grunerite, magnetite, hematite, stilpnomelane, ankerite, siderite, calcite, chalcedonic quartz in iron formations.

[2] The riebeckite granite known as ailsite, found on the island of Ailsa Craig in western Scotland, is prized for its use in the manufacture of curling stones.

The commonwealth's name is even taken from an Algonquian word for the Great Blue Hill, which got its color from this form of granite.