Magic ore is the raw form of magic steel. Even in its unrefined form, magic ore was considered to be valuable. The reason is simple. It is rare, and versatile.
Between magic ores and regular ores, there is one decisive point that sets them apart. Without exception, magic ores will only form around the vicinity of superior demons. The reason being, that when regular ores are exposed to dense concentrations of magic essence, they would slowly absorb it over a very long period of time.
Only when a great amount of magic essence accumulated will the regular ores finally undergo a change into magic ores. It is similar to how minerals can change.
Naturally, only powerful demons inhabit the locations with high levels of magic essence. Magic ores are hard to find in locations like the habitats of weak demons that adventurers defeat to earn pocket money. For magic ores to form, at the very least, it needs to be a place that B-ranked demons or above would live in.
Magic ore are already very difficult to obtain, but the amount of magic steel that could be extracted from that ore was only 3~5%. In other words, a lump of magic steel the size of a fist, was worth 20 times its weight in gold.