An essential difference among various stones is their degree of hardness.   Each gemstone can scratch a softer stone and can in turn be scratched by a harder stone.  The degree of hardness has been measured for over two centuries by a scale devised by an Austrian professor of minerology, Friedrich Mohs.  The Mohs scale classifies stones from the softest (1) to the hardest (10).  Diamond is the hardest stone and the softest are gypsum and soapstone, also called talc.  Here's a brief summary of some common stones:
Hardness    Characteristics                                        Stone
1                    Can be scratched with fingernail: Gypsum, Soapstone
2                    Can be scratched with fingernail: Rock Salt
3                    Can be scratched with knife or coin: Calcite
4                    Can be carved with knife or glass: Fluorite
5                    Can be carved with a knife: Apatite
6                    Can  be carved with glass or steel pen: Orthoclase
7                    Can scratch glass: Quartz
8                    Can scratch glass and quartz: Topaz
9                    Can scratch topaz and glass, and can itself be scratched only by diamond:
                                                             Corundum (blue = sapphire, red = ruby)
10                  Cannot itself be scratched, but can scratch all other gemstones: Diamond
Source: Healing Crystals and Gemstones by Dr. Flora Peschek-Bohmer and Gisela Schreiber.
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