Anything that's steely resembles the metal called steel in some way—it's either hard, strong, bright, or silvery. A sword, a shade of gray, and even a look can be steely.
You might describe the steely glint of the polished silverware on the table, or the steely blue of your cat's eyes. If someone questions you with a steely coldness, they are hard and emotionless—as cold as steel. You could also describe this person as "flinty" or "stony," other unbending adjectives. When steely was first used in the 16th century, it also described things literally "made of steel."