Lauren, I hope we haven’t chased you away, as we've simply been having some silly fun.
An easy way to tell between the two, although they are very similar in appearance, is noting that marcasite is a stone (actually pyrite and varietals, since ‘marcasite’ is a misnomer) and cut steel a metal alloy -- and so they're going to be treated differently.
The former will be cut like a stone and placed in a setting. While steel is going to be cut as a stud (like a nail head), so that it pierces the item and then secured via riveting on the reverse (if the purse allows for checking the back of the trim, this is one way to check).
And then there are those imitation cut steel metal strips that are simply stamped to look like individual studs.
If you look closely at cut steel (you may want to google), you’ll notice how round and multi-faceted the surface of the studs are, while marcasite has fewer facets and is most often rose cut (a point on top of the stone).
Now, if you look closely at your purse, you’ll be able to see those rose cut stones (see the points?) in their settings with beaded prongs. It’s quite beautiful, and I know you’ll do well with it.
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