くさび石いし

noun

keystone

In architecture, the central wedge-shaped stone at the top of an arch that locks the others in place. Also used figuratively for a crucial element.

See also: 要石

• アーチのくさび石いしが全ぜん体たいを支ささえている。

  The keystone of the arch supports the whole structure.

noun

sphene; titanite

A mineral, calcium titanium silicate, also known as titanite. Used in geology and mineralogy.

See also: チタン石

• くさび石いしはチタンの鉱こう石せき鉱こう物ぶつの一ひとつです。

  Sphene is one of the titanium ore minerals.