Haematite, chamosite and limonite. Egypt
Haematite (blue-white, highest reflectance) forms the central ooid, which has a poorly defined concentric banding. Haematite is present in other ooids, again following the concentric banding. Limonite (light grey, left top, right centre, bottom centre) is banded and incompletely infills voids (black) between the ooids, so forming the matrix. Dark grey areas are chamosite (top right).
Polished block, plane polarized light, x 40, air