One of only four elements with a unique last letter (6)

I believe the answer is:

cobalt

(Other definitions for cobalt that I've seen before include "Deep blue pigment" , "Metallic element used in blue/green pigments" , "A deep-blue colour (6)" , "kind of blue" , "Deep shade of 16 dn" .)

I've seen this clue in the Newsday.

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