Spheroidal Diorite of Shiraishi: Japan's Only Nationally Designated Chrysanthemum Stone
On the rice terraces of Inu-Sotoba in the western reaches of Shiraishi City, Miyagi Prefecture, lies a rock unlike any other in Japan. Cracked open and polished, its surface resolves into clusters of pale rosettes: concentric circles of minerals three to six centimeters across, arranged like the petals of chrysanthemum blossoms. Local people have called it kikumenseki, the chrysanthemum-face stone, for as long as anyone remembers. Geologists know it as spheroidal diorite, and it was placed under national protection on March 7, 1923, as the only deposit of its kind in Japan to receive that designation.
The same rock has been documented from only a handful of places on Earth. The most celebrated is the Mediterranean island of Corsica, Napoleon Bonaparte's birthplace, which gave the stone its international nickname: napoléonite, or "Napoleon stone." Outside Corsica, scattered occurrences are known from Sweden and parts of the United States. The Shiraishi deposit belongs to that small global club.
How the Pattern Forms
Diorite is a coarse-grained igneous rock that cools slowly underground from magma. What makes the Shiraishi stone unusual is the spheroidal habit: instead of forming the more familiar speckled texture of ordinary diorite, certain minerals crystallized in tight concentric shells around a central nucleus. According to records compiled by the Miyagi Prefectural Board of Education, each sphere has a hornblende core, a middle ring rich in plagioclase feldspar, and an outer band of biotite mica and hornblende combined. The minerals are not piled randomly; they radiate outward, which is precisely what produces the chrysanthemum-petal effect once a weathered face is exposed.
The conditions that cause magma to crystallize this way are still not fully understood. Most working hypotheses involve sudden chemical changes at the margins of a magma chamber, or fragments of older rock acting as seeds around which minerals built up in pulses. Whatever the mechanism, the result is rare enough that geology textbooks the world over reach for the Shiraishi or the Corsican specimens as illustrations.
A Dream of Onion Flowers: The 1595 Discovery
The local legend of how the stone first came to light dates to the late sixteenth century, the closing years of the warring states era. In Bunroku 4 (1595), two brothers named Heiemon and Yasuemon farmed land at Miya-no-Sawa in the hamlet of Inu-Sotoba. According to the account preserved by the Shiraishi City Board of Education, the elder brother dreamed for three nights running that onion flowers were blooming across one of their paddies. Unsettled by the repetition, the brothers dug into the field and uncovered a remarkable stone marked with chrysanthemum-like rosettes.
They enshrined the stone as a divine object and built a small shrine on the spot, calling it Kikumenseki-jinja, the Shrine of the Chrysanthemum-Face Stone. The shrine still stands today in the Ōtakasawa district, though its inner sanctuary is not open to the public. The place name "Kikumenseki" survives on local maps and signage as a quiet reminder of the discovery.
The Edo Period Stone Collector Who Named It
The first scientific notice of the Shiraishi rock came roughly two centuries later. Kinoki Sekitei (1724–1808) was an Edo-period collector of kiseki, or "curious stones": minerals, fossils, and oddities of natural form that fascinated a network of cultivated amateurs scattered across the Tokugawa realm. In his multi-volume catalog Unkonshi (Records of Cloud Roots), the first part of which appeared in 1773, Kinoki described a specimen from Shiraishi under the bewildered name nandamonda, roughly "what on earth is this thing?" The record, cited in the standard Japanese reference Genshoku Ganseki Zukan (A Color Illustrated Guide to Rocks), is generally counted as the earliest written account of this remarkable outcrop.
That a wandering antiquarian could puzzle over the rock so productively says something about the long Japanese tradition of careful natural observation, which fed directly into the country's modern earth sciences during the Meiji era. By the time the Imperial government's predecessor to the Agency for Cultural Affairs took up the question in the early twentieth century, the Shiraishi deposit's scientific value was already well established. National designation followed in 1923, with Shiraishi City taking over administration in June 1925.
What There Is to See, and Where
Visitors expecting to walk up to a dramatic cliff face will find the situation more modest. The protected outcrop itself lies across farmland in the Shirakawa-Inu-Sotoba and Ōtakasawa-Ōmachi districts, with collection or removal of stones strictly prohibited under the protection law. For most travelers, the best way to actually see the rock is at the Shiraishi Central Community Center (Shiraishi-shi Chūō Kōminkan), where a polished specimen is on permanent display. Staff at the reception desk will point the way; the case is open during regular hours, 8:30 in the morning to 5:15 in the afternoon, with photography allowed but no touching.
For those who want to make a small pilgrimage to the source country, a short drive into the Ōtakasawa hills leads past the Kikumenseki shrine and through landscape that still feels essentially agricultural. The Wanko-no-Ie roadside farm stand on Route 113 makes a sensible stop, with locally raised pork, the famous Shiraishi uumen noodles, and a small restaurant. The Ōtakasawa Community Center also keeps specimens for local educational use and can be contacted in advance.
Combining the Visit with Shiraishi's Other Sights
Shiraishi sits on the JR Tōhoku Main Line and the Tōhoku Shinkansen route, roughly fifty minutes south of Sendai by limited express. The city is best known as the castle town of the Katakura clan, retainers of the Date family who governed northern Honshu from Sendai for two and a half centuries. The reconstructed donjon of Shiraishi Castle, the preserved samurai quarter at the Katakura residence, and the warrior-courtyard gardens repay a half-day walk.
Lunch in Shiraishi means uumen, an oil-free wheat noodle developed locally in the seventeenth century and traditionally served in clear hot broth. Beyond the city, the Zaō mountain range rises to the west, a destination for hot springs, hiking in summer, and the celebrated juhyō (ice-clad trees) of February. The chrysanthemum stone is a quiet stop on a fuller day, not a stand-alone destination, but it has the pleasing quality of being entirely unlike anything else most travelers will have seen.
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Q&A
- Can I see the spheroidal diorite outcrop in the wild?
- The protected sites themselves are on farmland and are not set up for casual visitors; collection of any rock is forbidden under the cultural-property law. A polished specimen is displayed at the Shiraishi Central Community Center, which is the practical place to view it.
- Why is it called "chrysanthemum-face stone"?
- When the rock is weathered or polished, the concentric mineral spheres show as petal-like rosettes about the size of a coin. The pattern recalls the sixteen-petal chrysanthemum that is the Japanese imperial crest, hence the local name kikumenseki.
- Why is the Shiraishi deposit so important?
- Worldwide, spheroidal diorite has been recorded from only a few places, most famously Corsica. Within Japan, Shiraishi is the only locality designated as a national natural monument for this rock type, and the first to have been documented in writing — by the Edo-period collector Kinoki Sekitei in his 1773 catalog Unkonshi.
- Is the Kikumenseki Shrine open?
- The shrine grounds in Ōtakasawa can be approached, but the inner sanctuary where the stone is enshrined is not open to the public. The shrine commemorates the 1595 legend of brothers Heiemon and Yasuemon and is mainly of interest to those already drawn to the story.
- How do I reach the Shiraishi Central Community Center?
- It is about a seven-minute walk from JR Shiraishi Station on the Tōhoku Main Line, at 25-6 Aza-Terayashiki-mae. Visitors arriving at Shiraishi-Zaō Station on the Tōhoku Shinkansen will need a short taxi or bus ride into the older town center.
Basic Information
| Name | Spheroidal Diorite (Kyūjō Sen'ryokugan / 球状閃緑岩) — locally Kikumenseki |
|---|---|
| Designation | National Natural Monument (designated March 7, 1923) |
| Administering body | Shiraishi City (since June 3, 1925) |
| Protected site | Shirakawa-Inu-Sotoba and Ōtakasawa-Ōmachi, Shiraishi City, Miyagi Prefecture |
| Rock type | Spheroidal variant of diorite (holocrystalline granular structure); each sphere 3–6 cm across, composed of a hornblende core, plagioclase middle layer, and outer band of biotite mica and hornblende |
| Display location | Shiraishi Central Community Center, 25-6 Aza-Terayashiki-mae, Shiraishi City |
| Display hours | 8:30–17:15. Photography permitted; touching prohibited. Inquire at the reception desk. |
| Access | Approximately 7 minutes on foot from JR Shiraishi Station (Tōhoku Main Line); 10 minutes by car from Shiraishi Station or 20 minutes from Shiraishi IC (Tōhoku Expressway) |
| International occurrences | Most notably Corsica (the type of locality for napoléonite); also Sweden and the United States |
References
- National Cultural Property Database, Agency for Cultural Affairs — Spheroidal Diorite
- https://kunishitei.bunka.go.jp/bsys/maindetails/401/183
- Cultural Heritage Online (NII) — Spheroidal Diorite
- https://bunka.nii.ac.jp/heritages/detail/200986
- Shiraishi City — Designated Cultural Property: Spheroidal Diorite
- https://www.city.shiroishi.miyagi.jp/soshiki/30/374.html
- Miyagi Prefecture — Designated Cultural Property (Natural Monument): Spheroidal Diorite
- https://www.pref.miyagi.jp/soshiki/bunkazai/24senryoku.html
- Geological Society of Japan — Natural Monuments Tour (Miyagi)
- https://geosociety.jp/faq/content0558.html
Last verified: 2026.05.19