Thank you everyone for the kind words! I'm always sort of surprised when I get such a positive reaction to something Dr. Mario, as I don't remember it being that cool when it was new. Probably just not so cool among 9 year old boys...
It is indeed yarn, but 100% acrylic since it is meant to be a usable coaster. The yarn is all medium weight (#4) and most is Lion brand fun yarn since Jo-Ann had little skeins for $1 a few months ago when I got my first batch of supplies. The black is Red Heart super saver as I (correctly) guessed that I'd need more black than anything else and wanted a big skein.
Yet another advantage of mosaic stitch is it makes it easy to have a neat back, even when there are a lot of single stitches:
so I plan to just leave it as is.
I am *very* tempted to go get some floss or at least crochet cotton as there are so many more colors available, but I cannot justify the expense right now. Fortunately I like 8bit sprites, which play nicely with the colors available in yarn.