Sanukite

Sanukite formed from lava erupted during volcanic activity in the Seto Inland Sea region about 13 million years ago; it caps the mesa-like hills of northern Kagawa such as Goshikidai and Yashima. Known sources are very few: around Goshikidai, Kanayama and Shiroyama in Sakaide, the Nijōsan area on the Osaka–Nara border, and a few other places. The best-sounding sanukite comes only from a very limited area near Kokubudai on Goshikidai. Related "sanukitoid" rocks occur in the Setouchi volcanic belt from eastern Kyushu to Aichi Prefecture.

Sanukite is black, dense and hard, with fewer visible crystals (phenocrysts) than ordinary basalt or andesite. When broken it gives sharp edges and shell-like (conchoidal) fractures, and when tapped it gives a high, metallic, ringing sound with a pleasant resonance. Weathered pieces from Goshikidai can take unusual grey-to-white surface forms described as worm-eaten, rotten-wood, pine-cone or katsuobushi (dried-bonito) shapes.

Sanukite was used for stone tools in Kagawa from about 30,000 years ago until the end of the Yayoi period about 1,700 years ago, mainly as cutting blades, arrowheads and stone knives. At the Palaeolithic sites excavated during construction of the Seto Ōhashi Bridge (about 130,000 pieces at Yoshima-Seihō and about 350,000 at Hazashima), nearly 98 % of the stone tools and fragments were sanukite. In 1891 the German petrologist Weinschenk named the rock "Sanukit" after Sanuki; according to the Geological Society of Japan he studied samples that Naumann, the first professor of geology at Tokyo Imperial University, had taken back to Germany, while the Kagawa University Museum says Weinschenk came to Japan to study it. The English form "sanukite" became the international name. The Geological Society of Japan lists it as the representative rock of Kagawa Prefecture.

Because of its ringing tone, sanukite has long been called "kankan-ishi" in Sanuki, and since the Edo period it has also been known as "keiseki" (chime stone). It has been used for stone xylophones and other musical instruments and for signalling, and sanukite pieces are sold as local souvenirs.