Light crystal can be mined only from the crystal mining spots in the caves of the swamp zone if you are hr3 or lower. Once you get to Hr 4 and up they can be mined from almost any area during G-Rank quests and mined from mining spots 3 and 4 at the Pokke farm. Gypceros also give lightcrystals as quest rewards, especially if you break their head crest with a hammer or hunting horn.