The word quahog comes from the Narragansett poquauhock, meaning "hard clam." 

The Northern quahog (pronounced coe-hog) is a species of hard-shelled clam. They are found in intertidal and subtidal areas from Nova Scotia, Canada to Florida. The Native American name for the hard-shell clam (Mercenaria mercenaria) is Quahog (also spelled quahaug, quohog, and others) and the name is unique to the Cape and Islands as well as Rhode Island. 

The purple and white striations and patterns on the inside of the shell are created by minerals in the mud where the quahogs are dug.  Ocean quahogs are among the longest-lived marine organisms in the world. Off the U.S. East Coast, where the fishery takes place, ocean quahogs can live for at least 200 years. They grow very slowly and do not start to reproduce until around age 6, and do not reach a commercially harvestable size until about age 20. 

The oldest quahog that we know of died in 2006 when researchers opened it to verify its venerable age. A team of researchers has reported that Ming the Mollusk, the oldest clam ever found, is in fact 507-years-old, 102 years older than the previous estimate of its age. But that is as old as Ming will ever get.

“We thought they only came from oysters,” he says. They'd also never seen a purple pearl before. Pearls produced by quahog clams, are often button-shaped, like Overland's, and can be white, brown or purple.

Thank you, I gather that quathogs are like oysters but instead of clinging to rocks they live, and try to live very long lives, in the ocean sediment. Having no idea of their size, I wonder if they are like the shellfish pipis in Australia. They live under the sand in the tidal foreshore of our beaches.

Thanks @drpaladin. Pipis are about half that size, but obviously the same type of clam. Maybe a slightly different species.

Now I'll just say good night and clam up.

Just the facts:

QUAHOG Definition & Meaning - Merriam-Webster