Incidentally, the best official text description I know of is in Sorcerer, when you actually become a grue and visit a grue colony. IIRC, even that description is vague, but does cannonizesic] that they are large four-legged reptiles.
The unexamined emeralds cannot be both green and grue, since if they are grue and unexamined they are blue.
This is the grue,” said Dick, holding his lantern high. / Its light fell on a circle of skeletons, all perfect, each with its head toward a brass bowl in the center.
There was a sharp grue of ice in the air.