To see a world in a grain of sand and heaven in a wild flower, Hold infinity in the palm of your hand and eternity in an hour. -William Blake
Beach sand is some amazing stuff, and there's all kinds of it, from the black sands of Diamond Beach, Hawaii to the ultra-white sands of northwest Florida. Sand sculptors and geologists know much more than I would, but let's just say "sand" is no simple thing. The common ingredient is silica in the form of quartz, which makes up a majority of the sand on our beaches. Then it gets interesting.
Clay, shell fragments, minerals, limestone, gypsum, and even fossilized bones can be present in beach sand as well. Our local beach sand is fairly high in clay content because of erosion of sedimentary Rio Grand Mud, which has a reddish brown color. If you've ever seen the "clay turds" on the beach after the dredge deposits some sand on the beach, you're looking at pure Rio mud. Funny, "Isla Blanca" was rather misnamed because the sand is a brownish color, not pure white, simply because of the clay content.
Then there's the Laguna Madre, which also provides a bunch of black mud of its own due to rotting sea grass, along with a vast reservoir of calcite - a form of limestone. I have many very large calcite crystals from the Port Isabel channel area, some rods about 3-4 inches long or roses of several inches in diameter.
You'll notice some very old shells on the beach too, some turning a brownish color due to leaching and mineral deposition. These dark shells are probably over 10,000 years old when the island first formed. Mineralization is most likely from iron in the form of hematite; also, fossilized bones will also assume a darker color. One academic paper blames these minerals on the Trans Mexican Volcano Belt, if you're into petrography and words like "quartzofeldspathic"!
Yeah that was my reaction too, what-pathic? Go see a huge Mastodon tooth at the Beachcombers Museum on Pompano Street ... easier to pronounce ... it is very dark colored.
But a case can be made that the SPI area between Mansfield Cut and the Rio Grande has a very special kind of beach sand, like none found elsewhere in the world.