A crystal is a solid material whereby a batch of atoms forms a repeating pattern. They can be produced from the earth or manmade in a laboratory.
Crystals have the power to hold, convert and direct energies and for this reason they are used in radios and watches. Each crystal has a unique energy field with a unique frequency. Their unique energy field can influence our energy fields since we too are made of atoms and have our own energetic field. Each crystal can influence different areas of ourselves and our lives with their unique energy, colour and shape.
You can research crystals to find one with specific properties that are relevant to your individual concern and use it to align your personal vibration and energy field to that which the crystal can influence.
Aside of looking for a crystal with a specific energy and influence, you can also choose a crystal based on your gut. You can let your senses guide you to one as this could be your personal energy reaching out for what it needs rather than your mind telling you what you need.
Crystal healing is an alternative therapy whereby crystals are used to influence the human energy field. Crystals are used to absorb, convert and unblock energies that are stagnant, low or negative. It is an alternative therapy because it is not medically or scientifically proven. Crystal healing has been going for thousands of years and many people believe they have benefited from crystal power. It could be that some people are susceptible to energetic frequencies and can therefore feel and be influenced by them. For others it can be a placebo effect in which the power of belief is enough to bring about positive changes. The placebo effect is not fully understood but it is thought that if the brain believes in something, whether it is fake or not, it cannot differentiate and can therefore send signals to the body to heal. Either way, if crystals can be used as a tool to potentially bring about positive changes then there is no reason to dismiss them.