The Diamondof Granite

About 100 million years ago, magma cooled deep beneath what is now the Seto Inland Sea (瀬戸内海). Tectonic uplift later pushed this granite to the surface in Aji and Mure (牟礼), Kagawa, where it became known as Aji-ishi, the “Diamond of Granite.”

Aji-ishi is a fine-grained granite composed of quartz, feldspar, and biotite. Its tightly bonded crystals make it as hard as quartz with a Mohs hardness of 7, yet tough enough to allow intricate carving without chipping. With low water absorption and minimal iron content, its resistance to weathering makes it ideal for outdoor use.

When polished, Aji-ishi reveals a soft mottled pattern called fu (班), as if moisture were floating beneath the surface. This phenomenon is unique to Aji-ishi and has remained unexplained for centuries.

After cutting, each piece is polished and inspected for cracks or flaws. Only about 1% qualifies as the finest grade, reserved for temples, castle walls, gravestones, and major landmarks across Japan.

Today, Aji-ishi continues to evolve in contemporary architecture, public art, interiors, and experimental works led by the Aji Project.