How to craft it
Sky Milla building block that lights itself
It stands in for ordinary Cloud underfoot, but it glows on its own and throws small stars into the air above it once night falls. The stars are pure decoration. The glow is real light though, so a platform of this keeps a small area lit without a torch anywhere in sight.
where it comes from
Only certain secret seeds grow floating islands out of it, and on those worlds a share of the islands and sky lakes swap over to it. Everywhere else you craft it, which is what anyone who just wants the effect for a build ends up doing.