	THE LEARNING CENTER: Gems & Geology : Buying Tips : Care & Cleaning : FAQ : Links & Resources : About Us
	1.877.253.3747

Learning Center:

Gem Geology Gemstone Properties Diamonds Metals Birthstones Anniversary

Buying Tips:

Selecting Turquoise Selecting Pearls Selecting Opals

Learning Center: Gemstones & Geology

Rock & Mineral Properties | Other Geological Evaluation Tools

Rock & Mineral Properties

Ways in which geologists describe rocks & minerals - examples below:

Acicular	A needle-like arrangement of crystals; often applied to the description of very fine, radiating, hair-like crystals.
Crystal Face	One of the flat surfaces on a crystal.
Botryoidal Texture	A surface structure that resembles a bunch of grapes or looks bubbly (found on azurite and malachite specimens).
Conchoidal Fracture	The material breaks in a shell-like pattern or concentric circles, such as often seen on fractures in quartz and glass.
Crystal Cleavage	The tendency of a crystallized mineral to break along flat planes aligned with a zone of weak bonding within the crystalline structure of the mineral.
Crystal Fracture	The surface along which a material breaks. This is not the same as cleavage.
Dendritic Texture	Mineral shows flowery looking patterns or banding.
Density	A measurement of the mass of an object compared to its volume; often expressed in terms of grams per cubic centimeter.
Faceted Gemstone	A transparent gemstone that has been shaped and polished, with flat (often shiny) surfaces.
Fools Gold	A fanciful name for the mineral pyrite (iron pyrite). Early prospectors were often fooled by the yellow metallic color of pyrite and thought that they had discovered real gold.
Friability	The measure of how easy it is to crumble a material.
Glass	A solid in which the atoms are all in a chaotic distribution and not crystallized (such as Obsidian, volcanic glass).
Hardness	The ability of a material to scratch other materials or to be resistant to other materials trying to scratch it. See: Moh’s scale of hardness.
Luster	Refers to the shiny surface appearance of a material. Examples: Silky, metallic, non-metallic, vitreous (shiny/glassy looking).
Magnetic	A physical property in which a material is attracted to a magnet; such as magnetite.
Massive or Amorphous	A mineral having no specific shape.
Radioactive Half-life	The amount of time it takes for half of the atoms of a specific radioactive isotope to decay.
Rock	A solid, coherent mass of naturally occurring material made from one or more minerals.
Schiller	An optical effect, commonly observed with the mineral labradorite, where a play of colors suddenly appear when the specimen is oriented at a specific angle to a light source.
Silica	A compound of silicon and oxygen; usually refers to the mineral quartz but may also refer to opal.
Silica Tetrahedron	The basic building block of silicate minerals. Each silicon atom is surrounded by four oxygen atoms in order to create the silica tetrahedron.
Silicon	The element number 14; semi-metallic. It does not occur naturally in its pure form on the surface of the Earth.
Specific Gravity	The comparison of the density of a material with the density of water; usually expressed as a decimal number greater than 1 (because rocks and minerals rarely float)
Streak	The color of the powdered form of a mineral when it is rubbed against an unglazed, white porcelain tile.
Talc	A metamorphic mineral, typically white or pink, that is extremely soft. This mineral is ground to a very fine powder and when perfume is added to it, it becomes talcum powder.
Transparent	The optical condition where light goes through the material without any distortion. Color may be present, but if the material were to be finished as a flat, polished slab, you could read newsprint through it.
Vitreous Luster	A glassy luster such as found on Rose Quartz and Obsidian.

Source: Geologic Vocabulary - Cochise College

Left to Right:
Opal Cabachons (Australia)
Montana Agate
Coral Replaced by Agate

Gem photo courtesy of Roger Weller, Professor of Geology at Cochise College.

Top ˆ

Other Geological Evaluation Tools

Fracture: When a rock breaks roughly or unevenly this is referred to as the fracture surface or fracture plane. For example quartz (also glass) will fracture in a concoidal shape-much like curved concentric circles or shell-like in appearance. Obsidian (natural volcanic glass) also breaks with concoidal fracture.

Often you will notice that the rough edge of broken glass will have a similar concoidal fracture—be careful as this surface looks smooth and interesting, but it can still cut your skin!

Other types or forms of fracture include fibrous, splintery or hackly.

Cleavage: When a rock breaks along a plane or flat surface, this is referred to as the cleavage. Cleavage is a natural phenomenon and often is a reflection of the crystal surfaces present.

The angle between cleavage planes can also be used to determine the crystal structure such as with ortho- and clino-pyroxenes (90°) and amphiboles (120°).

For more information: Try a Google search with the term “Calcite mineral” to bring up available images of the mineral calcite using your web browser (images will appear in a separate window).

Crystals: Structure & composition
The study of crystals comprises several fields within geology including: crystallography, geochemistry, and igneous rocks: granites and volcanic rocks such as basalt, metamorphic rocks (schists, slates, shales, etc) and sedimentary rocks (limestone, chalk, sandstone, etc). If you are interested in more information, please check out the many website links shown on our links and resources page.

Top ˆ

Gem photo credits: All photos of gemstones and minerals are reproduced with kind permission of Roger Weller, Professor of Geology at Cochise College, AZ. Mr. Weller has many more beautiful photos of gemstones, minerals and rock examples. The Cochise College science and geology website is an excellent source of information for those that would like to learn more about the wonderful treasures and geologic evolution of the planet we live on.


QUESTIONS? Contact candice@cjfundesigns.com or call us at 1.877.253.3747.

Site Designed by Take Flight Graphics	Home : Privacy Policy : Shipping Policy : Return Policy : Contact Us : Sitemap	Copyright © 2006, CJ Designs, Inc., All Rights Reserved