Crystals are so unique and each one beautiful in it's own way. Some have different spots of color or other minerals mixed in, some have a cracked appearance, or odd shapes and markings. Like people, these little imperfections make each one unique, different, special and add character and charisma.
When you're trying to choose which crystal is right for you, tune into the energy of it and listen for the calling. If you're in a rock shop or are in front of many different crystals, simply start looking them over, holding them in your hands and opening yourself up to which particular crystal is calling you. Listen to your inner guidance. Sometimes you can feel the energy coming off of a stone. Sometimes your eyes will be drawn to one in particular. Sometimes you'll just intuitively know that for some reason this is the crystal that wants to work with you right now.
If you're shopping online, such as at ScentualGoddess.com, and are trying to figure out which type of crystals to choose (not the specific piece) use the same technique. Look through all the crystals available. Ask for the right crystals to speak to you. Then pay attention to which one(s) your eyes keep getting drawn to or which names keep popping out at you. Trust that the right crystal will be drawn to you at the right time in your life.