This is an extremely rare, very large crystal of the REE species Genthelvite. It is so rare that I am aware of only a handful from any location, notably MSH in Quebec, and of only a few other crystals from this locale which is now famous for other REE mineral species such as bastnaesite and parisite. It is the only one of quality, that I have seen for sale recently, in fact. Rightmost photo shows it backlit.