The color of stones is really just as important as what kind of stone it is. And when you pick a stone, as with any magick, it is important to know how this perticular stone, not the kind, but the individual stone, reacts to your own body chemistry, your feeling about the stone is always more important then the kind, or color. And make sure before buying your stones that you know that what you are thinking your buying, really is what it says it is. Alot of stores sell these magickal semi-precious stones.
Or you could just find stones in nature, preferably in a magickal spot you feel power.
Then, once you buy or find your stones, you must cleanse them. Sometimes they don't need cleansing when you find them in nature, but it's better to be absolutely sure that you don't get any negative energies mixed in with you magick. Or even if the other energies aren't negative, it's best to be sure that when you charge a stone, that's all it's charged with, it works best that way, and you can be sure that your magick is done purely.
I'v read many different methods for cleansing, so whatever has worked for you before, or what you trust will work, or whatever you'v read is best for whatever certain thing your planning to use your stone for, you should do.

White stones:
Use for spiritual guidance, moon magick , visions, divination, and dreams. White is ruled by the moon. Examples: Diamond, pearl, white opal, white chalcedony, white quartz, beryl. In modern america people cary white stones to attract good luck and fortune.

Quartz:
Comes in many colors and forms. Rock crystal, is a widely known form of this stone. Rocks with white quartz predominate in them would be in this class.

Diamonds:
Have been used in spellworkings to increase physical strength.
The diamond is not a love stone, but rather a gem of steadfastness and purification. The diamond is connected with fire. It figures, because what are called herkimer diamonds are really double-terminated quartz crystals and can be subsituted for diamonds in magickal workings. Diamonds promote inner searching and seeking, a balancing of the inner forces of the magician.

Pearl:
Is connected with water, spirit, and the moon. I have found that imitaion pearls work with dragon magick as well, should that be your purpose.

White Beryl:
A stone of moon and water, is a gem of energy, healing, and the psych.

The White Opal:
Is not entirely white, but has within it flashes of other colors. Devoloping psychic power, astal traveling, and working on past lives.

White Chalcedony:
Is useful for banishing fears and promoting calm.
It also protects against psychic attack and negative magick. It can both absorb, and repel energies.