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Emeralds are formed of a mineral called beryl whose chemical formula is a complex mix of beryllium, aluminum, silicon, and oxygen.Corundum also forms sapphire in many colors, which generally come from trace mixtures of iron, titanium, and chromium. The red color is caused by traces of chromium. Rubies are formed of a mineral called corundum, comprised of aluminum oxide.Graphite is also made of carbon atoms, but with a different arrangement-explaining why diamond is the hardest mineral and graphite (used in pencil lead) is one of the softest. Formed under extremely high pressure hundreds of miles underground, they are found in very few locations around the world. Diamonds, made of carbon atoms, are the hardest natural substance found on Earth.Precious gems are formed in different ways and composed of different materials, meaning their appearances vary vastly. There are a number of semiprecious gems, many quite beautiful, but diamonds, rubies, sapphires, and emeralds continue to qualify as "precious." (At one time, amethyst was also considered a precious gem, but large reserves were later found in Brazil, reducing its value.) Historically, gems have been divided into precious and semiprecious classes. It's only when they are cut and polished that they obtain the brilliance and luster that gives them their value. Uncut gems often look fairly ordinary–like rocks. Many minerals form beautiful crystals, but the most prized of all are gemstones. Oxides, sulfides, sulfates, carbonates, and halides are other major mineral classes. Silicates-including quartz, mica, olivine, and precious minerals such as emeralds-are the most common class of minerals, as well as the major components of most rocks. Rocks themselves are made of clusters or mixtures of minerals, and minerals and rocks affect landform development and form natural resources such as gold, tin, iron, marble, and granite. Geologists can tell a chalcedony from the arrangement and structure of its crystals. Chalcedonies include many types of cryptocrystalline quartz gems and feature a number of different colors.