The English names of equatorial hummingbirds often contain references to jewels. This one is particularly well chosen. He is fairly common on the west slope.
Note: Ridgely and Greenfield's "The Birds of Ecuador", 2001, has this as a separate species from the Blue-tailed Emerald (Chlorostilbon mellisugus). Opus has it as race melanorhynchus of that species. I don't know what the current situation is, but you won't find this in Opus under Chlorostilbon melanorhynchus, and I don't have the expertise to sort it all out, so I'll stick with Ridgely and Greenfield.