Usually red, but can be other colours. It is an octohedral crystal. The picture below is a close-up of the crystal on the left. It is about 2mm across.
Red spinel is often mistaken for ruby. The so-called Black Prince's Ruby in the Crown in the British Crown Jewels is in fact a large spinel. The Black Prince was the son of Edward III. Henry V wore this spinel in his helmet at Agincourt in 1415, possibly as a good luck charm.