Union County: Agate & Apophyllite Rockhounding in Somerset County, New Jersey

Plan your visit

Generally open to casual rockhounding

County

Somerset County

State

New Jersey

Nearest road

Valley Road

Postcode

07069

Coordinates

40.66022, -74.40913

Coordinates are approximate. Confirm the exact ground in the RockHoundR app, where land overlays show which agency manages the area.

Land & collecting status

Generally open to casual rockhounding

Most public-tagged spots sit on BLM, U.S. Forest Service, or other federal land where reasonable hand collecting of common rocks and minerals is allowed. Confirm posted rules and active mining claims before you dig.

Public-land rules vary by agency, season, and field office. The RockHoundR app pulls live BLM, USFS, NPS, and tribal overlays so you can see exactly which agency manages the ground at this spot.

Best time to visit Union County

Comfortable months (45 to 90°F highs)

Best months: March to December. Typical highs run 88°F in midsummer and 40°F in midwinter, with winter lows near 23°F.

Climate normals: NOAA 1991-2020 monthlies, PLAINFIELD station (4 mi away).

Geology at Union County

This spot sits on Orange Mountain Basalt, mapped as basalt that formed roughly 200 to 175 million years ago. Volcanic ground like this is where agate, jasper, and other chalcedony typically form, filling old gas pockets and fracture seams in the flow rock.

Mapped unit

Orange Mountain Basalt

Age

200–175 Ma (Early Jurassic)

Rock types

basalt

Elevation

220 ft

Geologic mapping via Macrostrat and the underlying state geological survey maps (CC BY 4.0). Bedrock at the surface can vary within a mapped unit.

Sources & verification

Spot details combine the public RockHoundR location dataset, normalized mineral labels, agency land-status checks in the app, and community submissions. Coordinates are approximate until verified in the field.

• New Jersey Geological and Water Survey
• Monmouth County Park System, fossil collecting rules
• Sterling Hill Mining Museum
• Community submissions reviewed against published agency rules