Some  basic  information and list some common terms relating to rocks, mierals 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 ofstones 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)