kojna -koj-ko'a- gismu

x1 is a corner/point/at-least-3-dimensional [solid] angle [shape/form] in/on x2, of material x3.

Also apex; a corner exists on three dimensions but need not be limited to points; it suggests a discontinuity in slope in some direction; i.e. in some planar cross-section. See also jipno, konju, bliku, fanmo, jganu, krasi.

In notes:

bliku
x1 is a block [3-dimensional shape/form] of material x2, surfaces/sides x3.
fanmo
x1 is an end/finish/termination of thing/process x2; [not necessarily implying completeness].
jganu
x1 is an angle [2-dimensional shape/form] from vertex x2 subtended by lateral [segment] x3.
jipno
x1 is a tip/point/vertex/extremity/end [0-dimension shape/form] on object x2 at locus x3.
konju
x1 is a cone [shape/form] of material x2 with vertex x3.
krasi
x1 (site/event) is a source/start/beginning/origin of x2 (object/event/process).
rokyko'a
r1=k1 is a cornerstone of structure k2 of stone type/composition k3=r2.