Beautiful, but what is it?

I've got an interesting volunteer growing in my yard. Okay, most people would call this a weed -- I would. I was in fact about to dig it out the other day, but realized...

...this is quite beautiful! Look at the silver edges along each leaf margin. Is this normal?

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I did a little bit of research using the interactive University of Missouri weed ID guide. (Like this plant, that guide is quite nice!)

I'm not certain, but this may be a Musk Thistle, Carduus nutans.

The silver leaf edges don't seem to be mentioned though, so I'm not certain. Why wouldn't they mention the silver edges?

Any ideas what this is?

I may have to wait until it flowers to find out -- and I will let it flower!

(If you know or have an educated guess, please let me know.)

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