What counts as a mineral is less settled than a textbook suggests. A mineral must pass five tests: it forms naturally, so steel is not a mineral; it is inorganic; it is usually solid at room temperature; it has an ordered internal structure that generally forms crystals; and it has a definite chemical composition that may vary only within limits. Yet reference guides still list some natural substances that lack a crystal structure, such as opal and liquid mercury, as minerals even though they do not meet every part of the definition. Obsidian, a volcanic glass that forms when lava cools so fast that its atoms cannot arrange into crystals, is called a mineraloid for the same reason.

Even the question of which mineral is most common has two answers. As of November 2026, quartz is the most common individual mineral, but the feldspar group as a whole is more prevalent, so the answer depends on whether you count a group or one mineral. Quartz is made of silicon dioxide, or silica, and when rocks weather away, what is left usually contains quartz, which is why the sand on most of the world's beaches is quartz. Quartz grains survive transport best because quartz is chemically inert, hard and not prone to cleavage, so it is often the most abundant grain in sandstone.

Gems are a different matter. They are mostly exceedingly rare minerals, unlike the slightly more than a dozen common minerals that make up most of what we see at Earth's surface. Silicate minerals make up most of the rocks we see at that surface. Many known minerals are rare and not familiar to collectors because they have no economic use, and as of November 2026 new minerals are still being discovered. Some elements, including gold, copper, platinum, sulfur and iron, occur naturally as minerals, but most minerals are chemical compounds of two or more elements.

Carbon shows how much structure matters. Diamond is made entirely of carbon, with each atom joined to four others by strong covalent bonds in a tight, even pattern. Graphite is the other mineral form of carbon, yet it is extremely soft. Diamond forms at the high temperatures and pressures of Earth's mantle, about 160 kilometers down, and most diamonds found have been carried up by deep-source volcanic eruptions. Graphite forms at roughly 75,000 pounds per square inch and 750 degrees Celsius in Earth's crust or upper mantle. It can form when coal seams are metamorphosed, because heat destroys the coal's organic molecules and leaves nearly pure carbon. Many people think diamonds form from coal, but geologists say that is not true.

The scale that ranks hardness was devised in 1812 by Friedrich Mohs, a German mineralogist, who chose ten minerals from very soft talc to very hard diamond. From softest to hardest they are talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond, numbered 1 to 10. A mineral scratched by number 5 has a hardness of 5 or less, while one that is not scratched has a hardness of 5 or more. Testing with several reference minerals narrows the estimate. The scale is not even: diamond is about four times harder than corundum, and its hardness is directional, so it is hardest parallel to its octahedral planes and softest parallel to its cubic planes.

Igneous rocks record how fast melt cooled. Magma rises because it is hotter and lighter than the surrounding rock, and if it stays below, slow cooling over thousands or millions of years lets the crystals grow large. Intrusive rock of this kind has a coarse texture, and granite, diorite, gabbro and peridotite are examples. Introductory geology textbooks say granite is the most abundant rock in the continental crust, and it shows up in the cores of many mountain ranges. Fast cooling leaves crystals little time to grow, so extrusive rocks are fine-grained or glassy. Trapped gas can leave a bubbly texture, as in pumice, which floats on water until it slowly becomes waterlogged.

Basalt is the other great igneous rock. It is dark and fine-grained, made mostly of plagioclase and pyroxene, and it underlies more of Earth's surface than any other rock type, including most of the ocean basins. Shield volcanoes are built from thousands of fluid flows that spread far and cool as thin sheets. As of November 2026, Mauna Loa, the largest shield volcano, rises 13,677 feet above sea level, and its top is over 28,000 feet above the deep ocean floor. All the Hawaiian Islands were built from basalt eruptions on the sea floor, and as of November 2026 the island of Hawaii is thought to be between 300,000 and 600,000 years old.

Sedimentary rock is made in steps. Weathering breaks rock into fragments, wind, water, ice or living things carry them away, and once enough sediment settles, the lowest layers are compacted so tightly that they form solid rock. Clastic rocks are named by grain size: shale is mostly clay, siltstone is silt-sized grains, sandstone is sand-sized, and conglomerate is pebbles surrounded by sand or mud. Sandstone grains are held together by a cementing material, and the sand reaches its basin by rivers, waves or wind. Coal forms from plant debris that piles up in oxygen-poor swamp water, where lack of oxygen keeps the debris from decaying, and about ten feet of debris becomes only one foot of coal.

Metamorphism changes rock without melting it. It creates new minerals, either by rearranging a rock's mineral components or by reactions with fluids that enter the rock. Contact metamorphism happens when hot igneous rock pushes into older rock and bakes it, changing its minerals without added pressure. Regional metamorphism can change shale into slate, then phyllite, then schist, and finally gneiss, a high-grade rock in which grains recrystallized under intense heat and pressure. Foliation, the striped look, forms when pressure squeezes flat minerals so they line up. But rocks made of minerals that are not flat, such as limestone, do not align no matter how much pressure is applied.

Dating all this rock takes care, and it begins with radiometric methods. Radiometric dating measures the age of rocks from the decay of long-lived radioactive isotopes, and it applies where a rock was last melted or disturbed enough to reset its radioactive elements. Earth's oldest rocks have been recycled and destroyed by plate tectonics, so scientists cannot find the exact age of the Earth directly from Earth rocks. A U.S. Geological Survey page from 2007 names the Acasta Gneisses in northwestern Canada, at 4.03 billion years, and the Isua rocks in West Greenland, at 3.7 to 3.8 billion years, as the oldest; newer finds may differ.

Those oldest rocks are not a primordial crust, because they are lava flows and sediments laid down in shallow water, which shows that Earth history began well before they formed. The Moon cannot answer the question alone either: the oldest dated Moon rocks are between 4.4 and 4.5 billion years old, though only a small number of rocks were returned by the six Apollo and three Luna missions. Every dated stone is a piece of a record that remains incomplete, and the earliest pages of it are gone from Earth.

Words to know

• mineraloid: not-quite mineral
• feldspar: common mineral group
• foliation: mineral striping
• isotopes: atom versions