gemmologist n.

"a precious stone" (especially when cut or polished), c. 1300, probably from Old French gemme (12c.), from Latin gemma "precious stone, jewel," originally "bud," from Proto-Italic *gebma- "bud, sprout," from PIE *geb-m- "sprout, bud" (source also of Lithuanian žembėti "to germinate, sprout," Old Church Slavonic prozebnoti "to germinate").

The two competing traditional etymologies trace it either to the root *gembh- "tooth, nail" [Watkins] or *gem- "'to press." De Vaan finds the second "semantically unconvincing" and leans toward the first despite the difficult sense connection.

Of persons, "a rare or excellent example (of something)" from late 13c. Alternative forms iemme, gimme persisted into 14c. and might represent a survival of Old English gimm "precious stone, gem, jewel," also "eye," which was borrowed directly from Latin gemma.

word-forming element indicating "branch of knowledge, science," now the usual form of -logy. Originally used c. 1800 in nonce formations (commonsensology, etc.), it gained legitimacy by influence of the proper formation in geology, mythology, etc., where the -o- is a stem vowel in the previous element.

The second element is prop[erly] -logy (-logue, etc.), the -o- belonging to the preceding element; but the accent makes the apparent element in E[nglish] to be -ology, which is hence often used as an independent word. [Century Dictionary]