Along with New Zealand and Canada, the Japanese archipelago is said to be one the world's three primary sources of iron sand, so it naturally follows that iron would have been made from iron sand since ancient times in Japan.

So, what is iron sand?

Iron sand is a material produced when titanium magnetite and ferro-titanium iron ore in volcanic stone such as granite and diorite—generated as magma cools in the depths of the earth at the upper mantle and the lower sections of the earth's crust—separates off as a result of weathering. The stone was produced between the Jurassic Period (approximately 200 million years ago) during the age of dinosaurs, and the Neocene Period (approximately 5 million years ago).

Magma is composed of fused or melted silicate minerals. It breaks through weak portions of the earth's crust and thickens as it rises. Crystals of various sorts precipitate in the process. As the composition of the remaining liquid is changed, the magma hardens and turns to volcanic stone. This stone, hardened magma produced in the depths of the earth, is the parent rock of titanium magnetite. It has acidic characteristics. Examples of this kind of rock include granite, granite-porphyry, and biotite granite. Given that it tends not to react with other substances in the earth's crust, masa iron sand has few impurities and is commonly used in the kera-oshi iron-manufacturing method. The parent rock for akome iron sand used in the zuku-oshi method, on the other hand, is thought to be produced through reactions with sedimentary rock when the magma is forced upward. Akome iron sand contains a comparatively high quantity of impurities and ferro-titanium iron ore, and exhibits basicity. Parent rock types include basalt, andesite, aplite, diorite, and granodiorite.

Titanium magnetite is formed by a combination of magnetite and Fe2TiO4. It is found along the coastlines of the Sanriku (an area comprising the three northernmost prefectures of the island of Honshu), Hokuriku (Fukui, Ishikawa, and Toyama prefectures), the San'in region (Shimane and Tottori prefectures), as well as northern Kyushu. Ferro-titanium iron ore is comprised of hematite (Fe2O3) and ilmenite (FeTiO3), with titanium particles in abundance. It is found in a wide range of areas, including Fukushima, Yamagata, and Akita prefectures, as well as the northern Kanto (the region around Tokyo), Tokai (around Nagoya), Kinki (Kyoto-Osaka-Kobe), the San'yo region (western Hyogo Prefecture, and south of the San'in region), and the islands of Shikoku and Kyushu.

Sample chemical composition of iron sand (mass %)

Geologically, the San'in region where the tatara were developed is younger than the San'yo side. It is formed by belts of titanium magnetite (a masa iron sand belt) whose main constituent is biotite granite. The geological features on the San'yo side of the mountains reveal the presence of ferro-titanium iron ore (an akome iron sand belt).

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