Cursed Ore is a crystalline ore found in Landia. It was considered "cursed" due to it making magic which was considered a blessing the gods unusable and can explode when exposed to magic. It poses a health hazard to miners who make heavy use of magic in mining making mining difficult in areas with high amounts of cursed ore.
However the real reason for its properties is that the ore absorbs magical energy rather than being cursed and explodes when the amount of energy absorbed exceeds the tolerance threshold. As discovered by Genia Maxwell, cursed ore is usable as a battery to power technologies such as the Little Susumu.
It was also shown to be a failure as a weapon against magically-enhanced armor. For example, Kazuya Souma thought to use the ore in an arrowhead. The ore did absorb the magical enchantment of the armor, but that only reduced the situation down to the level of the metallic protection of the armor against the arrow. The end result was no real damage to the armor by the arrow and wasted ore.
The former Elfrieden Kingdom was considered "rich" in cursed ore making mining of its already limited mineral resources hard.