When I go to a store to buy gems, I take some things into consideration. Many stones have a variety of qualities. I think about what I want the stone for. There are a variety of different ones to chose from, decided on the stone to suit your need.
Sometimes one will immediately catch your attention. If that happens you may choose that one.
Other times you will just see a collection of stones. Using your receptive hand (the one you don't write with) dig your hand in and sift through them. You should be left with a few in your hand. Put them aside and hold each one individually for a moment with your eyes closed. Does it have a comfortable feeling? Do you like the feeling you get from holding or rubbing it? These are the ones to buy. You may feel vibrations from stones and gems; there is life pulsing through them. Try to 'tune in' to their vibrations. It may take time but it's worth the wait.