Men-An-Tol

Men-An-Tol is believed to be Bronze Age (3500 years old) and consists of four stones. What makes it special is the rarity of standing stones with holes in them. there is speculation as to whether this is part of a larger stone circle or burial chamber and as always there are myths and stories suggesting it may be linked to fertility or curing of diseases such as rickets. Passing through the hole is supposed to be key to these beliefs, but I can find relatively little about this on the internet. 

The doughnut-shaped stone is best known for the traditional belief that it had the power to heal illnesses. To cure rickets and tuberculosis, naked children were passed through the hole three times and then dragged through the grass three times toward the east. Adults seeking cure from rheumatism or spinal troubles crawled through the hole nine times against the sun. Click here for source

The nearby Nine Maidens - although there were eleven stones clearly visible. 

And a single stone in a nearby field - Men Scryfa.