Rock of the Month

These quartz veins stand in relief of the host rock, a gray sandstone, which has weathered much faster than the quartz.  This specimen came from a dune, where chemical and physical weathering may quickly remove rocks weaker than quartz.

This sandstone has a narrow gap where a thin quartz vein has been removed by weathering.  Some pitting and removal is visible on the other veins as well.

US Silica quarried high purity quartz from this quarry to be crushed and sized for use in glass making, as a filter medium in swimming pools, aquariums, clean water treatment systems and wastewater treatment systems.  The quartz is an igneous intrusion 1.6 km long and 71 meters thick.

US Silica quarried high purity quartz from this quarry to be crushed and sized for use in glass making, as a filter medium in swimming pools, aquariums, clean water treatment systems and wastewater treatment systems.  The quartz is an igneous intrusion 1.6 km long and 71 meters thick.

This crystal was found in the marsh adjacent to Lieutenant Island in Wellfleet.  Although worn, the crystal faces are still discernable.

Chalcedony is a form of cryptocrystalline silica, the same chemical composition as quartz, but whose crystals are only visible through a petrographic microscope.  Chalcedony is formed as quartz precipitates from solutions in various types of rock.

This granite consists of pink feldspar and glassy quartz.  Both minerals crystallized from a magma rich in silica, alumina, and potassium.

Quartz veins cutting different rock types are commonplace on Wellfleet beaches. Sandstone can be scratched with a steel pocketknife, leaving a white powder. Quartz in harder that steel, and when scratched, a steel streak remains on the quartz as seen here.

The horizontal bedding represents the original layering of the sand due to water currents before transformation into sandstone.  The horizontal bedding has been cut by thin vertical quartz veins.

The thinner quartz vein has been offset slightly where it intersects the thicker vein, suggesting that the thinner vein was emplaced first.

The yellow-brown mineral veins are likely one or more iron oxide and/or hydroxide compounds:  hematite, limonite or goethite.  The quartz in these specimens was shattered by igneous activity, during which iron oxides were deposited and subsequently weathered into the ochrous crusts seen in these pebbles.

Due to their resistance to weathering and durability even to wave action, quartz boulders may retain an impressive size compared to other, less durable beach rocks.

Due to their resistance to weathering and durability even to wave action, quartz boulders may retain an impressive size compared to other, less durable beach rocks.