There are a selection of geology tutorials from introductory to advanced levels covering a range of topics which together form a free online textbook.
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Below is the abstract of an example tutorial.
Ooids are spherical or ellipsoid concretions of calcium carbonate, usually less than 2mm in diameter (Donahue, 1969; Tucker and Wright, 1990). There have been examples in the Neoprotozoic of ooids that are 16mm in diameter (Sumner, 1993), but all modern ooids are 2mm or less. The interior of an ooid is usually composed of a nucleus, which is surrounded by a cortex of calcite or aragonite crystals that are arranged radially, tangentially or randomly (Figure 1). These crystals are arranged in concentric lamina.