repost: Stone types and terms

[Guest guest]

Stone types and terms

 

 

Some basic information and list some common terms relating to

rocks, minerals and crystals .

 

Our main focus here is on the mystical spirits and healing

energy of crystals. A little bit of the more mundane information can

give us guidance . Rocks can be placed in three groups for convenience :

 

Igneous rocks :

 

Include Granite, rhyolite, obsidian, and basalt among others.

Igneous rocks are formed by the solidification of molten magma that

emerges via volcanic activity and emerges or erupts through vents or

fissures in the Earth's Crust The nature and properties of these

crystals vary greatly depending in part on the conditions under which

the magma solidified . fast cooling creates rocks that tend not to have

crystals slower cooling can give similar kinds of atoms trapped in the

magma time to find each other and form crystals . There are thousands of

different kinds of igneous rocks.

 

 

Sedimentary Rocks

Rocks such as limestone , sandstone, shales, halites coals,

and gypsum are formed by the accumulation of " sediments " these are

fine rock particles or fragments, skeletons of microscopic organisms

or minerals leached from rocks that have accumulated from weathering.

These sediments have been redeposited under water and were later

compressed in layers over time and are classed as either evaporative

(often water soluble ) or organic sedimentary rocks.

 

Metamorphic rocks

Rocks such as marble, slate , schist, gneiss and quartzite are

formed by the alteration of igneous and sedimentary rocks through heat

and/or pressure. These physical and/or chemical changes to rocks

maybe exemplified by the formation of marble from thermal changes

that have occurred to limestone.

 

What is the difference between a rock and a mineral?

A mineral is defined as a substance that must have all four of

these characteristics: It must be found in nature. It must not be made

of anything that has ever been alive (organic). It has the same

chemical makeup wherever it is found and its atoms are arranged in

regular patterns and form crystals. It is solid.

 

Rocks are sometimes defined as an aggregate or combination of

one or more minerals and the definition is extended to cover clay ,loose

sand and certain limestones.

 

Crystal

a crystal is a uniform body with a geometric lattice. the

varying structures of the lattice are the causes of the varying

physical properties of the crystals and therefore also of the minerals

and gems.

Piezoelectricity is that quality by which electricity and

sometimes light is produced by compression . Many cultures and spiritual

traditions have used crystals in ritual, often striking them to produce

flashes of visible light.

Pyroelectricity is that quality of an electrified state or

polarity which is produced by variation in temperature. Quartz has both

piezoelectric and pyroelectric properties which means that the

polarity of quartz crystal will change when subject to pressure or

heat as well as when held.

 

Crystal Terminology

 

Asterism

Shining lines that cross one another like rays from a star,

caused by

light reflected by a series of microscopic canals.

 

Chatoyancy

Bands of light and dark that vary in width and hue as the stone

is turned in the light.

 

Cleavage

A mineral's tendency to split when force is applied, leaving a flat

surface. This happens when there are layers of weakness within

the crystal.

 

Cryptocrystalline

Composed not of a single crystal crystal but of many extremely small

crystals, fused together.

 

Dichroism

The way different colors or shades can be seen according to the

angle

at which you look at the crystal.

 

Double Refraction

Light is split into two rays, producing a double image.

 

Inclusion

Any embedded foreign body, such as rutile crystals in Quartz, or

an ant in Amber.

 

Labradorescence

The colorful play of refracted light reflected from cleavage

planes.

 

Optical Lens

Refracted light is bent so as to enlarge, reduce, invert, or

otherwise distort an image.

 

 

 

 

Mohs Scale of Hardness

 

One way gemstones are classified is by hardness, both scratch

hardness and cutting resistance while the Mohs scale of hardness

developed by Frederich Mohs (1773-1839) is made somewhat obsolete by

modern technology an idea of the general hardness of a stone and

whether it is brittle helps you to protect your stones from damage and

can help with identification from softest to hardest the Mohs scale.

The Mohs Scale of Hardness has 10 classifications, 1 is the softest, and

10 is the hardest. The only mineral that is an exception to this is

mercury, which is liquid. The diamond is the hardest, rated 10. Your

fingernail is a 2, a pocket knife is about a 5-6, and a piece ofglass is

a 6-7.Each classification will scratch the one preceding it.

 

1 Talc

2 Gypsum

3 Calcite

4 Fluorite

5 Apatite

6 Orthoclase

7 Quartz

8 Topaz

9 Corundum

10 Diamond

 

 

Some minerals have varying hardness according to the direction

you may scratch them in, but typically this is either hardly detectable

or an exception to the rule. The following is a partial listing of

stones and their hardness classification:

 

1. Talc, Graphite: Can be scratched with a fingernail and by any

stone rated 2+

 

2. Gypsum, Bismuth, Lepidolite, Chlorite: Can be scratched with

afingernail and any stone rated 3+

 

3. Calcite, Celestite, Barite: Can be scratched with a knife and

anystone rated 4+

 

4. Fluorite, Malachite, Platinum: Can be scratched with a knife

and any stone rated 5+. Will scratch any stone rated 3-.

 

5 Apatite, Dioptase: Can be scratched with a knife and any stone

rated 6+. Will scratch any stone rated 4-

 

6. Feldspar, Pyrite, Amazonite, Hematite: Can be scratched with

a knife and any stone rated 7+. Will scratch any stone rated 5-.

 

7. Quartz, Tourmaline: Will scratch glass and any stone rated

6-. Can be scratched by stones 8+.

 

8. Topaz, Spinel: Will scratch glass and any stone rated 7-.

Can be scratched by stones 9-10.

 

9. Corundum (ruby, sapphire): Will scratch glass and any stone

rated 8-. Can be scratched by diamond.

 

10.Diamond: Will scratch glass and all stones 1-9

 

 

 

( Some of this is adapted from " The Handy Science Answer Book "

and from package notes from several little rock collection kits and

" Gemstones of the World " by Walter Schumann)

 

Peggy Jentoft