A mineral is a naturally occurring solid with a well-defined chemical composition and a definite atomic structure that has been formed by geological processes. Water, although it occurs naturally, is a liquid and, therefore, is not a mineral. Ice, on the other hand, when it forms in nature (not in your refrigerator), is a mineral. In fact, it is the most common mineral on the Earth’s surface.
A rock is an aggregate of mineral grains. A rock can be composed of several different minerals (e.g., granite, which contains the minerals quartz, orthoclase, and albite) or a single mineral (e.g., sandstone, which contains grains of only quartz).
A gem is a material that has been cut, polished or otherwise modified for decorative purposes. Most gems are fashioned from minerals (e.g., diamond, corundum, beryl), but they are often given different gem variety names depending on their colors ( e.g., ruby is red corundum, emerald is green beryl and amethyst is purple quartz).
The ideal characteristics of a gem are beauty, durability, and rarity. Gem value is usually determined by the "four Cs" — color, clarity, cut, and carat. The last referring to the size, measured in carats (1 carat = 0.2 gram). However, tradition and marketing are also important factors.
There are well over 4,000 officially recognized mineral species and as many as a hundred new ones are described each year.
Minerals, as the constituents of rocks, make up the solid earth on which we live. Minerals exposed on and near the Earth's surface provide essential nutrients and conditions for plant and animal life. Minerals have supported and been controlling influences on the development of and interactions between societies and civilizations throughout human history. Many wars and conquests have occurred because of the quest for mineral resources and the ages of ancient human civilization are actually defined by Man’s use of mineral materials (i.e., Stone Age, Bronze Age, Iron Age).
The Industrial Age brought increased demand for steel (iron) and metals such as manganese, chromium, nickel, tungsten, aluminum, and zinc, all of which are extracted from mined minerals. Buildings are made from materials like concrete, plaster, and glass which are made from minerals such as calcite, gypsum, and quartz. The Information Age and the Space Age are every bit as dependent on mineral resources.
Southern California has a tremendous variety of minerals, but finding places to collect them can be a challenge. Two books, which might be helpful, are Rockhounding California and Gem Trails of Southern California. The web can also be a good source of information. One particularly good site is DESERTUSA. If you are interested in doing more than just casual collecting and have an abiding interest in gems and minerals, consider joining a gem and mineral club. We have links to several local club websites on our Resources page. Finally, if you’d like to also support the Natural History Museum, consider joining our Gem & Mineral Council.
Meteorites are rocks from other planets or asteroids in our solar system. Some were excavated by impacts on the planet’s surface. Many others are the pieces of small planets that shattered during collisions with other planets! Meteorites often contain iron and other minerals that can be abundant in the earth's interior but rare at its surface. This can make it easier to distinguish them from normal rocks. Here are five helpful questions to answer about the rock you think might be a meteorite.
Is it magnetic?
While some meteorites are made of iron-nickel alloys, most meteorites contain at least small amounts of the metal. They should atract a magnet. Terrestrial rocks containing the mineral magnetite, and man-made industrial iron and slags will also attract a magnet.
Is there a fusion crust?
The 15 seconds of intense heat generated during a trip through the atmosphere will burn the outside of the meteorite, resulting in a thin glassy black coating called a fusion crust. Once on the ground the fusion crust will gradually erode away.
What is its shape?
A meteorite may develop an aerodynamic shape as parts of its surface melt and burn away — a process called ablation — as it moves through the atmosphere. The surfaces of meteorites are usually smooth and featureless but sometimes have shallow depressions resembling thumbprints in wet clay.
Does it have chondrules?
Chondrules are microscopic to marble sized spheres found in most stony meteorites. They were originally blebs of molten magma that froze as the newly formed solar sytem began to cool. They are ancient mineral grains and are not found on Earth. Recognizing chondrules takes practice, as there are various types of spherical grains in earth rocks as well. Learning to recognize them under a microscope is useful.
Does it show a Widmanstatten Pattern?
When cut, polished, and brushed with a dilute solution of acid, nearly all iron meteorites show a crissross texture called a Widmanstatten Pattern. The pattern is from the intergrowth of multiple iron-nickel minerals. This pattern will not be found in man-made iron as it does not contain any nickel.
For further information, we recommend the Meteorite Page of the Cosmochemistry research group at UCLA.