Cowslips

Cowslips are another flower that is guarded and well-loved by the fey. Also known in the west of England as Culver's Keys, they are thought to be keys to unlock the way to faery gold. According to Answers.com (http://www.answers.com/topic/cowslip), where we got the above image, they're also known as Fairy Cups.

In Medieval times, according to Wightdruids.com (http://www.answers.com/topic/cowslip), it was called Herb Peter and Key Flower, and frightened faeries were said to hide in it. It was also said to have the ability to split rocks with treasure inside.   

Bibliography: Faeries, by Brian Froud and Alan Lee.