It's some sort of polyester holographic knit, though not very stretchy as knits go, and not a 4-way stretch. It looked and felt like it would be a nightmare to work with, but it was actually really nice.
The pattern is a colour blocked Little Girl's Skater Dress by Kitschy Coo- the same as I used for my other Frozen dresses a few weeks ago. I lengthened the cape for this and added a snowflake ribbon trim from the amazing ribbon stash I inherited from my Grandma.
I posted a little bit more about the construction etc on Kollabora. But basically this was quick and easy and it looks so sweet. Some little girl is going to be really happy with this...!