Quick answer: yes, mostly in miniature. Long Island beach sand carries real garnet (the purple-pink grains) and magnetite (the black grains that stick to a magnet), and at Montauk the dark sand even includes a trace of slightly radioactive monazite. Bigger finds are glacier-delivered pebbles: quartz, jasper, granite and gneiss dragged down from New England and the Hudson Highlands during the last ice age. Faceting-grade gems are not what you’ll find; genuine minerals are.
Why Long Island has “foreign” rocks at all
Long Island has no bedrock you can see. The whole island is a pile of sand, gravel and boulders bulldozed south by glaciers and dumped as moraines about 20,000 years ago. So every pebble on a Long Island beach came from somewhere else, mostly Connecticut, Massachusetts, and the Hudson Valley. We cover that story in How Long Island Was Built by Ice.
Purple sand = garnet
Look at the wrack line after a storm and you’ll sometimes see streaks of purple-to-pink sand. That color is garnet, eroded out of metamorphic rocks in New England. New York State Parks describes the sand at Robert Moses State Park as a mix of quartz, garnet, magnetite and shell, part of Jones Beach as quartz with garnet and magnetite, and the north side of Napeague and Hither Hills along Napeague Bay as having a “purplish hue” from magnetite and garnet (NY State Parks blog).
Garnet is heavier than quartz, so waves and wind sort it: the light quartz gets carried off and the heavy grains concentrate in dark streaks. Geologists call that a placer. See Placer deposits.
Black sand that follows a magnet
Run a magnet in a plastic bag through dark beach sand and it will come out bearded with black grains. That’s magnetite, an iron oxide. Colonial-era Long Islanders actually mined these sands for iron (East Hampton Star). A sample from Montauk Point tested at about 25 percent magnetite.
The rare fact: Montauk’s sand is (slightly) radioactive
An East Hampton Star story followed a local who noticed his Geiger counter jumping on Montauk sand: normal background was about 50 clicks per minute, but it hit 700 near Big Reed Pond and 800 on separated black sand. Stony Brook geologist Gilbert Hanson pointed to monazite, a rare-earth phosphate that carries thorium, mixed in with the garnet and magnetite. A 1961 study found these heavy minerals made up 2 to 8 percent of beach sand from Westhampton to East Hampton. Hanson wasn’t worried and takes students there on field trips; his one caveat: “I wouldn’t eat it” (East Hampton Star).
What bigger stones can you find?
- Quartz pebbles, milky white to clear, the most durable survivors of the glacial trip.
- Red and yellow jasper and chert.
- Garnet-studded gneiss and schist, sometimes with visible dark-red garnets in the rock.
- Granite and pegmatite with pink feldspar and flakes of mica.
- Petrified wood, rare but real; we have a Long Island specimen, and the backstory is in How Petrified Wood Ended Up on Long Island.
Tips
- Go after winter storms, which strip light sand and expose heavy-mineral streaks and gravel beds.
- Bring a magnet in a zip bag and a 10x loupe for garnet grains.
- Check state park rules before collecting; many parks prohibit removing natural materials.
FAQ
Is a clear, glassy beach pebble a diamond? On Long Island, it is almost certainly quartz. Quartz is real, but it is not diamond. See Quartz or diamond?
Related reading
- Garnet: not one mineral but a whole group
- Magnetite: the mineral that is actually magnetic
- Rockhounding the Northeast
Related pieces in the shop
- Petrified Wood Specimen, Long Island, New York
- Golden Mica with Tourmaline (the same New England pegmatite minerals the glaciers carried here)
- New Jersey / New York Mineral Specimen Lot
Sources: New York State Parks blog, “Sand: The Beaches’ Hidden Treasures”; East Hampton Star, “Black Sands of Montauk Go Beep Beep.”
