Under UV light, clean sand appears purple or black. Some minerals, such as calciumcarbonate in seashells, glow blue, as does a shovel handle in the picture above.
They would leave subtle clues in the isotopic composition of the limestone by shifting the ratios of light to heavy carbon and oxygen atoms in its calciumcarbonate.
Coccolithophores are microorganisms that are covered in plate-like shells made out of calciumcarbonate (chalk). The chalky shells reflect light, turning the ocean a milky turquoise color.