Mystery of the Black Diamonds (Vintage Signet Paperback) by Phyllis A. Whitney

Puzzling clues hinder Angie and Mark as they search for ghost town treasure.

When Angie and Mark first look at the treasure map given them by the old prospector, they are unable to figure out its meaning or even decide whether or not it is a hoax. But when they persuade their parents to visit the ghost town of Blossom, one clue after another comes to light.

Strange howlings wake them at night, strange eyes watch them as they walk the deserted streets, and it becomes evident that more than one person is hunting for the treasure.