coin / quoin are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
coin: (noun) a flat metal piece (usually a disc) used as money. (verb) make up.
quoin: (noun) expandable metal or wooden wedge used by printers to lock up a form within a chase. (noun) the keystone of an arch.
coin: k·OY·n
quoin: NA
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)