It seems there are strong indications that this is probably the case. But, only time will tell!

Well thus far nothing else seems to come forward as being keys to that forgotten box. I'd consider that spool of colored string to be just as legit a key!

hmmmm I could see that being a key...

For a second there I was about to scream spoiler. XD

I could see that as a possibility. Maybe if the mane six meet other ponies like them, element wise, and are given little tokens of that. They could be keys. It'd be pretty sweet watching them turn into keys by magic :3

Many people think this. You're not alone.

Ditto I think it might be the key I mean everypony saw the rainbow in Rarity's eyes and the thread glow at the end right?

If they didn't they must be blind, because when watching something people blink significantly less, and the likelihood of blinking at BOTH of those times is pretty damn slim.

You make her look so cute :)

I stand by my theory that the box is like the Precursor Stone from Jak II. Opening it is, generally, a bad a idea for the continued existence of the world. Unless opened under the correct conditions. As for what is in it? I believe the instructions for the creation of artifacts like the Elements of Harmony are within.

'Course, I only know about the box from second hand info, fragmented at that. I have no idea of the story surrounding it.

That's my theory

I support that canon, had the same idea after watching the show

Beautiful illustration of it

I think everypony agrees that it's a key.
Beautiful art, atryl. Coc looks very lovely.

I love Coco pommel and your art so much - so adorable!

when I saw it shimmer in that ep I screamed KEEEEEY!!

Lots of folks have come to the same conclusion about that spool of thread.