noun
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
アーチのくさび石が全体を支えている。
The keystone of the arch supports the whole structure.
See also: チタン石
くさび石はチタンの鉱石鉱物の一つです。
Sphene is one of the titanium ore minerals.
要石 also means 'keystone' but is more commonly used in figurative contexts, such as 'the linchpin of a plan'. くさび石 is more literal and architectural.
チタン石 is another name for titanite/sphene, using the katakana loanword for titanium. くさび石 is the native Japanese term.
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.