What gives individual stones their characteristics?
Different stones have different energetic properties. For example, a Tigers Eye can aid those seeking clarity, while Lapis Lazuli is said to expand our awareness and help us attune to our intuition. Rose Quartz is calming and sometimes referred to as the stone of gentle love. Many of the books available contain lists of crystals and what each one means. These meanings are simply interpretations of the energy each crystal carries. Some may consider a crystal with a fresh, lively, citrus feel to it to give zest and promote optimism, and therefore help to reduce depression.
Can the wrong crystal do me harm?
As with many objects used for divination and healing, a crystal can only do harm in the wrong hands. A crystal healer only uses a crystal as a tool. A great crystal does not make a great crystal healer, just as a great paintbrush does not make a great painter.
You do need to take some care if you are looking for a crystal healer. If you cannot get a recommendation from a previous client, speak to the healer before accepting treatment. If you don't feel you would trust them in some everyday matter, neither should you trust them with your well-being.
Use your intuition and you cannot go far wrong. If one day you feel a crystal no longer belongs in your home, it may be time to give it away. As you change you may find that you want different crystals around you, although many will stay with you happily for a long time.