An ore sensor is a small item made of gold, glass, gemstones, and an enchantment that makes it react to certain ores. Despite the special materials used to make one, it is not the most expensive item ever. At least the common ones are not expensive. Miners use these to help them find a certain ore they want. The glass circle in the middle will glow a shade of blue, green, or red depending on the ore that is nearby. An intelligent miner knows to keep a list with him to write down what colors the object produces around different ores.
There are other sensors similar to the ore sensor, such as special ore sensors that detect ores with magical properties. A trinket sensor reacts when there is a trinket nearby but since trinkets are so rare, the sensor doesn't help much and is usually expensive in whatever shop sells it.
The first ore sensor was created by a miner who wanted an easier way to locate certain ores. He had some knowledge of magic. When he brought the first ore sensor to the mine and used it, his fellow miners noticed it and wanted one as well. He created more and even taught a few how to make one. Eventually they became a popular tool among the miners.