The heart chakra is the fourth chakra, and is represented by the color green. In some people who have developed the ability to be unconditionally loving, the heart chakra becomes a rosy pink. This is the chakra of love, romance, relationships and communication.
If your heart chakra is healthy, you will both be able to empathize with others as well as accept love and friendship into your own life. You will have good friends who you get along with, and will be able to form loving relationships. You will also be compassionate towards yourself.
If your heart chakra is unhealthy, you will have trouble maintaining friendships and finding love. It might be that you feel lonely or are hard on yourself. It might also be that you are obsessive and domineering, or closed off, 'cold hearted'.
To help open your heart chakra, try working with green stones such as jade or aventurine. To be kinder and more loving towards yourself try rose quartz or mangano calcite. See a beautiful pink flower opening at your heart. Rose and geranium essential oil can help here. This will help draw friendship and romance into your world.
If the problem is that you are jealous or possessive it's time to cool things down. Interestingly, because this is the central chakra, you also use green to bring it down as well - your aim here is to focus more on the love, and less on the control. You need to learn to trust. You may sometimes feel too sensitive. At these times visualise the pink flower gently closing for a while, but don't leave it closed, it is important we all learn to work from the heart.
The Subtle Body by Cyndi Dale is an amazing encyclopedia of the human energy field, including an excellent section on the chakras.
The Seven Chakras