In geology, rock is a naturally occurring aggregate of minerals and/or mineraloids. The Earth's lithosphere is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic. Petrology is the scientific study of rocks.

Iron oxides and carbonates play a large part in many sedimentary rocks and are especially important as coloring agents. The red sands and limestones, for example, which are so abundant, contain small amounts of iron(III) oxide (hematite), which in a finely divided state gives a red hue to all rocks in which it is present. Limonite and goethite, on the other hand, makes rocks yellow or brown; manganese oxides, asphalt and other carbonaceous substances are the cause of the black color of many sediments. Bluish tints result sometimes from the presence of phosphates or of fluorite; while green is most frequently seen in rocks which contain glauconite or chlorite.

If the quartz from natural crystalline stone (such as quartz, beryl, calcite, obsidian, or amethyst), it may display the natural colouring and structure of the mineral from which it was fashioned. Some authors advise students to place a sigil, seal, or talsimanic emblem beneath a clear sphere, but most do not. Most authors suggest that the work of crystal gazing should be undertaken in a dimly-lit and quiet room, so as to foster visions and more easily allow the onset of a trance state.

Film made by Dan Alexoae

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