Imagine that a chocolate chip cookie is a rock. You can see the chips in every bite, and that is how a rock works too. The flour, sugar, butter and chocolate chips would be the minerals that make it up. Geologists define a rock as a mix of one or more minerals, and granite, basalt, limestone and sandstone are common rocks. Minerals are made by nature and are never alive, and they are the starting ingredients in just about every part of daily life, from smartphones to roads, homes and even food.

What counts as a mineral? A mineral must pass five tests. It forms naturally, so steel is not a mineral, and it is inorganic. It is usually solid at room temperature, and it has an ordered inside that generally forms crystals. It also has a definite chemical makeup that may vary only within limits. A substance that fails even one test is left out, no matter how much it looks like a mineral. Geologists name six common minerals as examples: quartz, feldspar, mica, amphibole, olivine and calcite. Gems, in contrast, are mostly very rare minerals.

How do you tell minerals apart? Friedrich Mohs, a German mineralogist, made a hardness scale in 1812, and he chose ten minerals, from very soft talc to very hard diamond. The ten, from softest to hardest, are talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond. The scale ranks minerals by what can scratch what. A mineral that is scratched by number 5 has a hardness of 5 or less, and one that is not scratched has a hardness of 5 or more. Testing with several of the ten narrows the guess.

You can use the scale at home with things from around the house. A fingernail has a Mohs hardness of about 2 to 2.5, a copper sheet is about 3, and glass is about 4 to 7. Mica minerals form flat, sheet-like crystals that peel apart into smooth flakes. Mica is so soft, at 2.5 on the Mohs scale, that a fingernail can scratch it. Talc, which is number 1 on the scale, is softer still, and it is a very soft mineral. Diamond, which is number 10, is the hardest natural substance known.

Quartz is one of the most common minerals on Earth. It is made of silicon dioxide, also called silica, and it has a Mohs hardness of 7. It comes in many colors, such as purple amethyst, gray smoky quartz, pink rose quartz and yellow-green citrine. When rocks weather away, what is left usually contains quartz, which is why the sand on most of the world's beaches is quartz. Fluorite, in purple, green, yellow and blue, is often mistaken for quartz, but it is softer, with a hardness of 4.

Graphite and diamond are the two mineral forms of carbon, yet graphite is extremely soft and diamond is the hardest known natural substance. Diamond is made entirely of carbon, and each carbon atom is joined to four others by strong bonds in a very tight, even pattern. That pattern makes diamond one of the most durable substances known. Many people think diamonds form from coal because both contain carbon, but geologists say that is not true.

Rocks tell how they were made. Granite forms when magma is trapped deep inside the Earth and cools over thousands or millions of years, so its mineral grains have time to grow large. Obsidian forms when lava cools so fast that its atoms cannot arrange into crystals. Pumice forms when gas bubbles are trapped in frothy magma that cools quickly, and the holes in pumice are those bubbles. Slow cooling grows big crystals, and fast cooling grows tiny ones.

Sedimentary rocks form from pieces of older rocks or from the remains of once-living things. They build up as deposits on Earth's surface and often show layers. Wind and rain slowly break big rocks into smaller pieces, and those pieces can end up as new rock. Sandstone, for example, is a rock made of sand grains stuck together. Some rocks also hold the remains of once-living things, such as shells and plants, and they keep a record of life from long ago.

Limestone is a sedimentary rock made mostly of the mineral calcite, and it usually forms in clear, calm, warm, shallow seas. So limestone found on land probably means the area was once under water. Calcite is easily scratched with a knife, and it fizzes when touched with acid because it contains carbonate. Most seashells are made of calcite or related minerals, and some limestone is made from the shells and skeletons of sea animals.

Metamorphic rocks form when rocks are squeezed and heated deep in the Earth. They do not melt, and the change makes denser, more compact rock. Heat and pressure can change a soft rock called shale into slate, and gneiss is a metamorphic rock with stripes or bands. Marble forms when limestone is changed by the heat and pressure of metamorphism. It is mostly the mineral calcite, and its calcite crystals interlock.

What a rock is made of, the shapes of its grains and how the grains fit together are clues that let geologists read the rock's history. Over very long times, one kind of rock can turn into another, and the never-ending change of rocks from one type into another is called the rock cycle. Every stone you pick up is one stop on that cycle, and a careful reader can learn to find its story.

Words to know

• geologists: Earth scientists
• magma: melted underground rock
• inorganic: never alive
• graphite: soft carbon
• obsidian: volcanic glass
• metamorphism: rock changing