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
The keystone of the arch supports the whole structure.
A mineral, calcium titanium silicate, also known as titanite. Used in geology and mineralogy.
See also: チタン石
Sphene is one of the titanium ore minerals.
Standard kanji-kana mixed spelling. The kanji 石 is clear, and くさび is written in kana.
Full kanji spelling using 楔 for 'wedge'. Less common and may be harder to read.
Compound of くさび (wedge) and 石 (stone), referring to the wedge-shaped stone used as a keystone. The mineral sense comes from the wedge-shaped crystals of sphene.