Sky crystals were crystals often used by sky pirates during the First Age of Flight. Sky crystals glowed when near another crystal, and sparked when struck together. Sky crystals could even spark when wet, making them useful in the rain. In Clash of the Sky Galleons, Quint uses his to light an ironwood beacon, a trick he learnt from Storm Weezit. They are also the gift given to librarian knights elect by Ulbus Vespius, the Professor of Light.
Maris also once made a mosaic out of sky crystals which her father produced during his preliminary experiments in the Ancient Laboratory. These ones came in four colours: red, yellow, green and purple. For an explanation of the differences between these two kinds of sky crystal, click here.