CeeLo -- NYC Jeweler Outraged ... We Don't Sell Fake Garbage

CeeLo was NOT scammed into buying a fake designer watch for $34,000 -- despite a "watch expert" calling it a hideous counterfeit -- so says the jewelry company that crafted the piece ... and now wants a public apology.

Avianne & Co. fired off a cease-and-desist letter to the guy behind the popular Instagram account FakeWatchBusta -- because FWB recently posted a pic of CeeLo's custom 20-carat diamond-studded Panerai from Avianne, and outed it as a bad "fake."

FWB went so far as to add the following caption: "I really hope #CeeLoGreen said “HELL NO!” when he saw the ugliest custom fake #Panerai watch in the world ... The crown guard must have been removed with a hammer and then reattached upside down."

Avianne demanded the CeeLo post be taken down immediately and replaced with an apology ... as well as a comment saying the watch is in fact 100% real.

The photo has since been deleted from Instagram, but so far ... no apology's been posted.

Fun fact: Avianne is the same company that made Ciara's 15-carat diamond engagement ring.