Agate - Agate

Japanese: メノウ(瑪瑙) - メノウ

It is a collection of very fine quartz crystals. It is semi-transparent and mainly occurs in clumps. Its composition is SiO ₂ , its Mohs hardness is 7, its specific gravity is 2.60, and its refractive index is 1.53. It is sometimes classified as chalcedony when it is relatively uniform in color and as agate when it has stripes, but generally both are referred to as agate. It is found all over the world, but because it shows shell-like exfoliation and the pieces have thin, sharp edges, it has been used in North Asia and India to make stone arrowheads and small stone blades, and beautiful colored pieces have been widely processed into ornaments since ancient times.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

石英のごく微細な結晶が集合したものをいう。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。

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

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