มรกต [มอระกด] น. ชื่อรัตนะอย่างหนึ่งในจำพวกนพรัตน์ มีสีเขียว.Etymonline.com has this to say about the origin of emerald:
"The name of a type of precious stone, one of the nine gems, green in color." (my translation)
c.1300, from O.Fr. emeraude, from M.L. esmaraldus, from L. smaragdus, from Gk. smaragdos "green gem," from Sem. baraq "shine"... Skt. maragdam "emerald" is from the same source...To sum up, we have English emerald coming from Greek smaragdos, related to the Sanskrit root maragdam. The Sanskrit has another form (or perhaps another transliteration, I'm not sure)--marakatam. The etymologies given in RID are woefully incomplete, so it's not surprising they don't acknowledge the Indic source of the word, but it's a pretty obvious connection between marakatam and มรกต. And thus between มรกต and emerald.