agate

...It is semi-transparent and is mainly mined in lumps. Its composition is SiO ₂ , its Mohs hardness is 7, its specific gravity is 2.60, and its refractive index is 1.53. Those with a relatively uniform and plain color are sometimes classified as chalcedony, and those with stripes as agate, but generally both are called agate. It is found all over the world, but because it shows shell-like exfoliation and the pieces have thin and sharp edges, it has been used in North Asia and India to make stone arrowheads and small stone blades, and those with beautiful colors have been widely processed into ornaments since ancient times. ...

*Some terminology explanations that mention "agate" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…そのため半透明状となり，主として塊状で産出する。成分SiO₂，モース硬度7，比重2.60，屈折率1.53で，色が比較的一様で無地なものを玉髄(カルセドニーchalcedony)，縞目のあるものをメノウ(アゲートagate)と分類することもあるが，一般には両方をいっしょにして〈メノウ〉と称している。世界各地に産するが，貝殻状剝離を示し，剝片は薄く鋭利な縁をもつため，北アジアやインドなどでは石鏃や小型の石刃の材料とされたほか，色の美しいものは古くから装身具として多く加工された。…

※「agate」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報