cobalt
鈷 centers on the metallic element cobalt. It is a specialized character used for this chemical element.
鈷 combines 金 (metal) with 古, which likely contributes the sound コ. It was created as a character for the element cobalt, following the pattern of using the metal radical for metallic elements.
The metal radical 金 on the left tells you this is a metal, and the right side 古 (old) hints at an old metal: cobalt has been used since ancient times in pigments and alloys.
For コ, imagine a cobalt-colored coat: coat -> コ, and the metal鈷 is used to make that deep blue coat of paint.
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trident vajra
single-pronged vajra; tokko; dokko; copper or iron implement, pointed at both ends, used in esoteric Buddhist rituals
ground stone tool from the late Jomon period (named after its similarity to the Buddhist tokko)
ground stone tool from the late Jomon period
three-pronged vajra flung by Kukai on his return from China to determine the Holy Land of Esoteric Buddhism