Tokachi-ishi

Shirataki obsidian formed about 2.2 million years ago, when renewed volcanic activity followed a large eruption about 3 million years ago; rapidly chilled outer parts of viscous lava became obsidian, while slowly cooled interiors became rhyolite. The main source area is Akaishiyama in Shirataki; at the Tokachi-Ishizawa lava the obsidian layer is about 7 m thick.

Under the microscope the rock is colorless glass with crystallites (globulites, margarites and trichites) and occasional quartz, plagioclase, biotite or hornblende crystals; white spherulites and banded flow structure may be visible. It breaks with a conchoidal fracture into sharp flakes. It is distinguished from pitchstone, a related volcanic glass, by its low water content (usually under 1%, versus about 4–10%).

Obsidian is brittle and chips or breaks easily on impact, and with a hardness of about 5 to 5.5 it scratches easily, so it suits earrings, pendants and brooches better than rings or bracelets. Household dust (hardness about 7) is harder than obsidian, so wipe it with a lint-free cloth instead of scrubbing it dry. Inclusions can expand when heated, so avoid mechanical cleaning systems and use mild soap, a soft brush and warm water.

The word 黒曜石 (kokuyōseki) appears in Kiuchi Sekitei's Unkonshi (1801) and was adopted as the translation of "obsidian" by Wada Tsunashirō in 1878. In 1846 the explorer Matsuura Takeshirō recorded that the same stone was called Monbetsu-ishi along the Yūbetsu River and Tokachi-ishi in the east; in the Meiji era, crafts made from Tokachi obsidian spread the name Tokachi-ishi across Hokkaido. Older names include anchi (anji) in Ainu, kokugyokuseki and kurozuishō. Shirataki obsidian was used for stone tools from the Paleolithic (artifacts probably about 30,000 to 15,000 years old), and was carried as far as Sakhalin in the north and Yamagata and Niigata in the south; artifacts from the Shirataki site group were designated a National Treasure in 2023, and Shirataki Geopark was certified as a Japanese Geopark in 2010.