Marble is limestone that has been cooked and squeezed by a mountain. The veins are the record of that journey, and no two slabs of the same block ever agree about them.
Each slab in this library is generated in the browser: a base field, a family of veins drawn as repeating linear gradients at complementary angles, a turbulence layer for tooth, and a specular sweep whose strength follows the polish setting. The Carrara will look like Carrara because of the angle and spacing of the veins, not because anyone photographed it.
That is roughly how the real thing works too. Quarrymen identify a stone by the direction of its bedding long before anyone thought to name it.