I know that this word is used to describe a variation of an emerald, although many dictionaries do not include this word. One that does is Merriam-Webster:
Middle English smaragde, from Latin smaragdus, from Greek smaragdos, of Semitic origin; akin to Akkadian barraqtu gemstone
What is the etymology of this word smaragd? Is the word emerald directly related to it? There is a superficial resemblance between the two words.
The definition posted hints at a semitic origin, which sounds likely - one (relatively archaic) Hebrew word for emerald is izmargad (אזמרגד), a close similarity.
However, Hebrew Wikitionary claims that the word has only been in use in Hebrew since the fifth century AD, placing it after the Greek usages, and likely to have been influenced by the Greek, rather than influencing it.
The same source claims that the ultimate root for the word resides in Sanskrit marakata, or "green", which seems to be corroborated by some online Sanskrit dictionaries whose veracity I cannot evaluate:
marakata n. an emerald [ confer, compare Greek ; Latin smaragdus.]
As to how smaragdos* evolved into emerald, Etymonline gives a good overview:
Greek smaragdos -> Latin smaragdus -> Medieval Latin esmeraldus -> Old French esmeraude -> French emeraude -> English emerald.
The same source also claims that the road from Sanskrit marakata to Greek smaragdos passed through the Semitic root baraq (shine), which can be found in the other Hebrew word for emerald, bareqet, but I'm not familiar enough with the phonetic shifts between Sanskrit and Semitic languages to say if that's likely.
Latin introduces words into English in two ways -- directly and circuitously via Old French. Thus the Latin word smaragdus remains in Biblical translations from the Vulgate, like Wycliffe's. In Wycliffe's Revelation, the green foundation stone of heaven is smaragdus, not emerald. Likewise, in scientific contexts, we have the Smaragdine Table (a translation of Tabula Smaragdina), a medieval-era book on alchemy. In mineralogy, smaragdite is a bright green silicate mineral.
According to the OED, the two forms entered the language at about the same time. Smaragdus (as smaragd) in 1272; emerald (as emeraundis) in 1300. One can only speculate on why the latter became popular and the former became obsolete.
