coriaceus is the Latin, not the English
vowel-digraphThe -eous of coriaceous is three letters and one weak syllable. Written by ear, one of the three tends to go missing.
heard ascoriaceus
spelledcoriaceous
saidko-ree-AY-shus
English writes coriaceous, with an o.
In Latin the adjective genuinely is coriaceus. English adds the o when it takes these words in, because -ous is the English adjective ending and -us is not: Latin famosus became famous, Latin nervosus became nervous. Coriaceous is the same adjustment applied to the same kind of word.
Also on this leaf
coriaseus corriaceous coriaseous coreaceous ceriaceous
All six are on the same page. All 6 spellings of coriaceous.