Rocks, Minerals, Gems

Perhaps no place is better to see and learn from rocks than the Grand Canyon. The exposed rock layers tell about the land and life forms in the past, going as far back as almost 2 billion years. Aside from the very bottom, the layers are sedimentary.

They were formed by constant pressure crushing tiny particles, such as sand, silt, or the bodies of small sea animals, together. The colors of each layer give clues about the minerals that make up that rock.

These horizontal rock ledges are Tapeats Sandstone. This layer was at the bottom of an ancient sea, whose sands were compressed into hard rock. Tapeats Sandstone is so hard to erode that it forms a platform. Other rock layers sit on top of it.

Rocks aren’t found just on Earth, of course. Rocks make up some other planets, as well as asteroids and comets that float through space. Sometimes bits break off these “parent” objects and come our way. Most burn up in Earth’s atmosphere, but some crash-land on the surface.

This huge rock, called Ahnighito, is actually just one chunk from a gigantic meteorite that broke apart before slamming into Greenland about 10,000 years ago. It’s 34 tons of iron, and it’s ancient—about 4.5 billion years old, almost as old as the sun. 

As super-hot gases cooled in the early solar system, they condensed into mineral solids. Different minerals, like calcium and iron, began to condense at different times and formed different objects. A meteorite’s minerals can tell us how old it is and what “parent” it came from.

You’re atop Mount Etna, the highest volcano in Europe and one of the most active volcanoes in the world. It’s been erupting almost constantly for thousands of years, spewing ash, gas, and smoke into the air and lava down its sides.

Lava—called magma when it’s below the ground—is hot melted rock. As it cools on the surface, it becomes hard, solid rock. In this way, volcanoes like Mount Etna are rock makers.
 

Lava brings up elements from below. As each element cools, its atoms arrange in a definite, repeating pattern, forming crystals. Minerals form. Most rocks are a mixture of minerals. Rock formed from lava, like this black rock, is called igneous (meaning “fire”).

Here at the Royal Belgian Institute of Natural Sciences, you can see ancient plants and animals that aren’t really plants and animals. They’re rock. The rock fossils here preserve traces of life forms that existed long ago.

Some are the remains of prehistoric trees found near the Belgian city of Hoegaarden. Some are the fossils of mammals from the Messel Pit in Germany. The process of fossilization can occur in a variety of ways.    

Do these look like parts of purplish trees to you? This is petrified wood, which is not wood but rock. Once they died, the cells of these ancient trees were gradually replaced by minerals that kept the structure of the tree tissue.

This little creature is Kopidodon macrognathus. Millions of years ago it fell into a deep lake and was buried under mud. A lack of oxygen and bacteria helped preserve its remains. Unlike many fossils, soft parts were also preserved. Can you see the outline of its fur?

You’re in the Hall of Geology, Gems, and Minerals of the Smithsonian Institution in Washington, D.C. Minerals are solid earth materials with crystal structures, and so are most gems. What is the difference between the two?

Most gems are mineral crystals that have been cut at specific angles to best reveal the mineral’s color and brilliance, and then highly polished. Gems are valued for their beauty and used in jewelry. Diamonds, rubies, and emeralds are examples of gems. Can you name others?

Aquamarine is a variety of the mineral beryl. The Dom Pedro is the largest aquamarine gem known in the world. It was cut from an even larger crystal found in Brazil. Because of the precise way it was cut, the gem appears to glow from within.

Topaz is a mineral that comes in a variety of colors, as you see here. That brings up an interesting fact: not only can gems from the same mineral have different colors, but different gems can have similar colors.

Quartz is a common mineral that comes in many varieties and colors, including lavender amethyst, pink rose quartz, and gold citrine.

Cut and polished in China, this quartz sphere—a veritable crystal ball!—weighs 106.75 lbs. (48.5 kg), and measures 12.9 inches (32.7 cm) in diameter.

Are you wondering where all the colors of the minerals come from? The answer is light. Remember that visible light has all the colors of the rainbow. The atoms of different mineral elements absorb certain colors in light and reflect others.

The colors they reflect are the colors you see.

Minerals that contain the element copper are blue and green. They include deep blue azurite, blue-green chrysocolla, and bright green malachite.

Can you tell how these minerals are arranged? They’re arranged in the colors of the rainbow, or the visible spectrum of light: violet/purple, blue, green, yellow, orange, red.