It is purely a decorative block that adds some nice touches to the structures it appears in.
Some blocks of Lunastone appear in the Lunic Garden surrounding the Lunic Altar. There are only a few there, though.
Lunastone is used as a roofing block for some of the buildings of the villages of The Hunger. There is more Lunastone here, since there are usually several buildings in a village.
Though it is used to help build the Hungers' houses, it is not offered in a trade by The Hunger. Since Lunastone is a stone-based Vethean block, it is unobtainable without a way to get a pickaxe in Vethea (whether through cheats or legitimately through another mod).