STONES & MINERALS

How To

Identify rocks and minerals

Mission

Successfully identify rocks and minerals

Briefing

A little knowledge of rocks and minerals will help you identify everything from jewelry to fossils.

While reading this article may not turn you into a geologist, it should give you the basic information needed to begin.

Your best bet is finding a thorough field guide to rocks and minerals. You can always try your hand at amateur geology using similar evaluative techniques.

Your first lesson: minerals are the building blocks for rocks. This means a mineral is a rock, but a rock is not a mineral—a rock is a composite of minerals.

Animal? Vegetable? Mineral...
These are the main properties geologists use to evaluate a mineral.

• Luster: The two primary types are metallic and non-metallic. A metallic mineral reflects light. Other luster markers are greasy, glassy and pearly.
• Transparency: How well light passes through an object. There are three degrees: transparent, translucent and opaque
• Density: Simply put, the weight. A mineral like graphite would be on the lighter end of the scale, with platinum and native gold at the heaviest.
• Hardness: Moh's Hardness Scale is the standard geologists use, with diamond being 10 at the hardest end of the scale and talc being number one, and softest. If you can scratch something with your fingernail it would rank 2.5 on the scale.
• Cleavage and Fracture: Some minerals cleave (or break) quite easily and along smooth lines. Others, such as, quartz and malachite are said to "fracture,"and break either with splintery, curved, or jagged edges.
• Crystal form: Crystals can develop in a variety of standard shapes and there are six typical crystal systems (isometric, tetragonal, hexagonal, orthorhombic, monoclinic, triclinic) to look for.
• Color: This is the least reliable and usually the last detail studied by geologists as some minerals may contain impurities that change color without changing their basic properties.

These are the most common identifiers that a geologist would use as they apply to the broadest range of minerals. Other properties that a geologist might look at would include, effervesence, streak, flouresence, crystal form and magnetism.

Change Rocks!
There are three major categories for classifying rocks: sedimentary, igneous and metamorphic.

• Sedimentary rocks give a geologic history lesson with their layers. For example: sand made from quartz forms into sandstone; clay minerals compress into shale.
• Igneous rocks are made from once hot, molten material that has solidified. The rocks are either made from lava that has erupted from a volcano on the earth's surface, or from magma, which is lava that exists at or below the earth's surface.
• Metamorphic rocks are formed when conditions underground, such as fluid, strain, heat, and/or pressure cause rocks to change or metamorphose.

History Detectives Tips

• If your sample is possibly valuable, don't do any physical testing (such as using hydrochloric acid, or a mineral hammer or even your fingernail), as these may cause permanent damage.
• Answer a series of questions, geologists have created flowcharts for this.
• Control your imagination: Always assume your sample is the most common option, not the rarest.