Mystery of the Black Diamonds by Phyllis A. Whitney (1974-05-07)

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 waken them at night, strange eyes watch them as they walk the deserted streets. It becomes evident that more than one person is hunting for the treasure, and there are those who want to hinder Angie and Mark in their search. Angie meets Jinx Kobler, a girl her own age, living with her grandparents in the crumbling town. Jinx loves the old town and resents outsiders. She is oddly secretive about her mother and father, adding one more question to those which puzzle Angie. Who or what is Abednego? What are the black diamonds and where are they hidden? Why did the old prospecter make no use of the treasure himself? What queer animal haunts these empty streets at night and steals from the newcomers?