Why were some cobalt ores historically called kobold ores?

Historically, German miners applied the term "kobold" to cobalt-bearing ores because they considered the mineral to be worthless and troublesome.1

In German folklore, a kobold is a mischievous spirit or goblin associated with caves and mines. Miners believed that these mountain goblins had either spoiled the silver ores they were mining or had stolen the silver from the ore entirely. Because the presence of these minerals was associated with such perceived trickery, the ores were named after the spirits blamed for the interference.123

The name eventually evolved into "cobalt" as the metallic element was identified and integrated into the language of science.1