"I know it when I see it" tends to be the most common responses to the question "What is Steampunk?"
The Ministry of Peculiar Occurrences says:
"Steampunk is an inspired movement of creativity and imagination. With a backdrop of either Victorian England or America’s Wild West at hand, modern technologies are re-imagined and realized as elaborate works of art, fashion, and mechanics. If Jules Verne or H.G. Wells were writing their science fiction today, it would be considered “steampunk.”
When I think of Steampunk, I imagine Victorians dreaming about the future - primarily in terms of technology. What if a person from the Victorian era time traveled to the future, and then upon return, tried to recreate the technology? It's about innovation, adventuring, and tinkering.
Things to consider for in a Steampunk clothing/jewelry:
1. Color. Contrary to what some say, Steampunk is not "what happened when Goths discovered brown." Yes, many things you see in a Google search for "Steampunk clothes" fall into the khaki-to-chocolate spectrum, but don't be limited by that stereotype! Rich, saturated tones and darling pastels can both work for Steampunk outfits.
2. Hardware. One of the things I think makes an outfit look more "Steampunk-y" is the addition of some hardware. Whether this be in the form of grommets, rivets, gears, or buckles, a bit of industrial edge keeps a look from being too Victorian. Also, mixing metals (silver, brass, copper, gold) is a great way to reference that cobbled-together-but-ingenius vibe.
3. Motif. As in many other styles, there are certain themes that act as visual shorthand - clocks, gears, keys, goggles, octopi, and airships - are common motifs in Steampunk.
Still not quite sure if a look is Steampunk? Here are four looks I've assembled for you that nod to Steampunk in varying degrees. If you're interested in a full-on look, I highly recommend looking at Clockwork Couture.