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Hematite
Wide swaths of red soil often indicate the presence of hematite, also known as red iron oxide. In Georgia, for example, red soil is abundant because of weathering that occurs in granite and gneiss rocks. The minerals left behind after weathering include iron, aluminum and silica. Oxidized iron causes soil to appear red. Likewise, in the Grand Canyon, hematite-rich sandstone is responsible for vibrant red banding.
Spessartine
Sometimes rocks contain small pockets of reddish minerals, such as spessartine, a type of garnet. Violet-red spessartines have been found in rhyolite rocks in Colorado and Maine. Manganese impurities (abbreviation Mn) are typically responsible for spessartine's brilliant red color. Spessartine's chemical formula is Mn(II)3Al2(SiO4)3.
Corundum
It would be unusual to find a hillside covered in rubies, but if you did you would be seeing the effects of chromium in corundum crystals. Corundum crystals contain aluminum and oxygen molecules that form a crystalline structure, Al2O3. When an impurity such as chromium (Cr) bonds with the corundum crystal's structure, then the result is a red ruby. The chemical formula for ruby is Al2O3:Cr.
Cinnabar
If the reddish color you observe is a vein-filling mineral near alkaline hot springs or recent volcanic activity, then you may have found cinnabar. Cinnabar, also called mercury sulfide, sometimes resembles quartz in its crystalline structure; however, its color ranges from bright scarlet to brick red. The chemical formula for cinnabar is HgS.