Has anybody heard of a rock that turns red when exposed to water, and when exposed to air turns white?

I found this small piece of a rock, with a smooth side similar to a gem, and another side that looked like it may have been part of a steel or other heavy metal. A friend of mine had dug it up, and over time it turned white when exposed to air, and eventually a deep purple. It was extremely hard, hard enough to scratch a railway tie. Has anybody heard of a mineral or stone that does this? Back in high-school I asked my physics teacher (who was originally a geologist) and he couldn't identify it.