It's often called a "pond damselfly" or a "narrow-winged damselfly."

We spotted this brilliant blue damselfly on a Great Valley gum plant (Grindelia camporum) near the Sciences Laboratory Building at the University of California, Davis.

It's a male coenagrionid damselfly, said Lynn Kimsey, director of the Bohart Museum of Entomology and professor of entomology at UC Davis. She knows her insects: she has seven million specimens in the Bohart (plus a few live ones in the "petting zoo").

The damselfly sparkled like a blue diamond as it foraged on the gum plant. 

An entomological treasure, an Odonato gem, a sliver of blue in a thicket of green.