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Fuji-Hakone-Izu NP, Tokyo
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Some Kōga stone is light enough to float, yet geologists describe it as porous rhyolite lava—not loose pumice—with flattened bubble spaces. This rare stone is found on Niijima and Italy’s Lipari Island.
Kōga stone formed in Niijima’s 886–887 CE Mukaiyama eruption, which built much of the island’s southern half. Old chronicles even describe the eruption as creating a new island.
After a devastating village fire in 1870, Niijima residents built Kōga-stone storehouses to protect their valuables. Stone sheds and walls later helped turn homesteads into fire-resistant compounds.
Niijima’s Kōga-stone Moyai statues aren’t simply named after Easter Island’s Moai: “moyai” means helping one another in the island’s dialect. The figures began as a local tourism idea after the 1964 Tokyo Olympics.
A new Niijima sea salt uses Kōga stone as insulation in its flat cooking kiln, instead of the firebrick normally used. Its maker intentionally mixes crystal sizes so fine grains dissolve first and larger ones follow.
Islanders once called the light, water-floating rock “kabu-ishi.” In 1913, mining professor Watanabe Wataru promoted it for export as 抗火石—“fire-resisting stone”—a name highlighting its heat resistance.
Early 20th-century rules allowed islanders to quarry Kōga stone. During slack farming periods, people cut blocks at the quarry and carried them home on their backs—building walls and a distinctive stone-built streetscape.
Cut stone from Ishiyama did not simply roll down to the sea: workers sent it by a two-stage ropeway to Torinoshima, off the Mamashita coast, where boats loaded it for shipment.
Kōga stone has a second life as glass: its crushed volcanic material supplies the silica used in Niijima glass, known for its distinctive olive-green color.
A Meiji-era account of Niijima records Kōga stone used for goldfish bowls, fireplaces and roof slabs. The island’s lightweight stone roofs were so distinctive that visiting observers remarked on them.
