You do not need to know the exact properties of every stone in order to buy one, although this may help. It is more important to be receptive and allow yourself to be drawn to a stone that will have meaning for you. If you are in a shop simply stand in front of a group of crystals, close your eyes and relax, and when you open them choose the one you feel most drawn to.
When choosing a crystal from an internet site it is not possible to touch the crystal, so you need to take your intuition one stage further. Browse through the pictures of the various crystals and note which section most appeals to you. Spend some time browsing through that section. You may find yourself excited by the picture of a particular crystal, with a powerful urge to touch and feel it and a sense of frustration that you cannot. If you feel that you very much want that crystal to be a part of your life, it is likely that on some level it has something to offer you. For example, you may feel drawn to a rose quartz crystal because you need something in your life that can help you develop a love of yourself.
You may see many beautiful crystals but none that compel you to buy them. As with all purchases relating to spiritual growth intent is crucial, and you may need to acknowledge that the time isn’t right. If you buy crystals purely because that is what you set out to do then your crystals may lack meaning, and you are essentially starting a rock collection. There is nothing wrong with collecting rocks. If nothing strongly appeals to you, accept that the time is not right and come back on another day when perhaps a crystal will plead with you, like a gorgeous puppy, to take it home with you.
In our personal development it is sometimes difficult to know what we need. We may see clearly when helping others with their lives but, as our opinions are coloured by what we think of ourselves, find it very hard to be objective when looking at our own. We may like to think we are loving all the time, but in some situations we may be sarcastic, or we may think we are forgiving, but in reality find it hard to talk to a person who has wronged us. Because of this, our conscious selves are not always best at choosing crystals. You may set out to buy a citrine crystal, having read that it can help dissipate negative energy, but be prepared to have your unconscious lead you to a different crystal. The unconscious or higher self doesn’t get involved with life’s day-to-day clutter and thus can get straight down to the business of selecting the most appropriate crystal for you. This is why the best approach to choosing a crystal is to quieten your mind and simply do what feels right.