Minerals, Gems and Precious Stones by Chemical Group

All minerals—and the gemstones that are made from them—can be classified into groups based on their chemical composition. This taxonomy features 175 selections, combining some of the most iconic and well-known gems and minerals with a few lesser-known specimens. Many popular gems are actually varieties of the same mineral. When that occurs, the "parent species" is noted beneath the mineral name. For instance, agate is a form of chalcedony, which itself is a variety of quartz. Similarly, rubies and sapphires are both varieties of corundum.

While some minerals share the same chemical composition—like the many forms of silicon dioxide (SiO₂) or silica—they can appear dramatically different due to trace elements such as iron, which affect color. Outward appearance is also shaped by factors like heat, pressure, crystalline structure, and inclusions of gas or water. For example, amethyst owes its purple hue to iron impurities and natural radiation exposure.

Some gemstones are classified as mineraloids, which lack an ordered internal crystal structure. Opal, for example, is an amorphous form of silica and typically contains 5–10% water. Another category, organics, includes biologically produced substances like pearl that are valued for their beauty and rarity.

Diamond, the hardest mineral, is made from a single element, carbon. Its crystal brilliance is due to its highly ordered internal structure of atoms.