Delaware County officials believe they have just bought a gem for use as a new park.

Mineral Hill, long known to rock collectors and museums for the unusual geological specimens found there, was purchased for $650,000 from Elwyn, a 130-year-old agency serving the disabled, county officials announced Tuesday.

Located along Baltimore Pike, the 46.2-acre site is next to other undeveloped tracts in Middletown and Upper Providence Townships, bringing the total to about 100 acres of open space.

Christine Fizzano Cannon, a Delaware County Council member, said the area was important as a "natural resource, a watershed buffer, and a geological treasure."

The heavily wooded area is marked by steep slopes and rock outcroppings. Materials recovered from the land are in museum collections around the world, officials said.

The long-abandoned Crump's Quarry used to mine serpentine, the green building stone used in many area houses, on the tract. A large pavilion from a defunct Boy Scout camp is still located on the property, which also once housed a pig farm, according to county officials.

Mineral collectors have been going there for more than a century, said Howell Bosbyshell, professor of geology at West Chester University.

Bosbyshell called Mineral Hill a "unique" geological area. The main bedrock in the area, ultramafic rock, is part of the Earth's upper mantle, pushed to the surface by tectonic activity.

"As a result of that, there are these cool minerals that occur there," Bosbyshell said.

There are still opportunities to find unusual minerals, but, he said, collectors will probably have to wait until erosion brings new specimens to the surface.

"What is easily accessible has been found," Bosbyshell said.

Fizzano Cannon said the county would install trails in the park over the next two years.

The property was acquired with money and help from the Natural Lands Trust, the Pennsylvania Department of Conservation and Natural Resources, and Middletown Township.

There may be a wrinkle in the county's plans for a larger park, however.

The status of the nearby Lavin tract in Upper Providence, which township officials have said they would like to sell for housing development, might still be up in the air. The property was purchased with bond money set aside to acquire open space. Open-space advocates and neighbors have opposed development of the area, which included wetlands.