The different between a character set and a graphic set

It sort of depends on your graphics pack, but the DF wiki has the default images/colors for each stone, as well as their uses.

http://df.magmawiki.com/index.php/DF2010:The_Non-Dwarf's_Guide_to_Rock

*EDIT*

Since you changed your question to refer specifically to the Phoebus graphics set, a quick clarification. In Dwarf Fortress, the graphics set does not include any information about the colors representing different items in that game, such as rock types. The Phoebus graphics set is just an image that contains all the tiles used in the game. The colors are set in the init file, and are changed independently of the graphics set. For example, the Matrix Tile Set asks the user the change the default colors, but it has to specify them for the user to change in the init file. There are some versions of Dwarf Fortress that come with a graphics set pre installed, and these will sometimes change the default colors to match the style of the set, as well as some other settings.

Unless you have personally changed the colors, or are using a special version of Dwarf Fortress, the colors will be the default, and can be found in the link I originally posted.

A full list and description is available at the df wiki.

In order from worst to best, with their additions to item quality:

• Nothing = 1x
• - = 2x
• + = 3x
• * = 4x
• ≡ = 5x
• ☼ = 12x
• Artifact items, (ones with a unique name) = 10x in the latest version, 120x in 0.28

The page for the previous version is far more detailed, but some information may be out of date.