It's important to be grounded enabling our bodies to connect with the energies of the earth, creating a clear path for them to rise into the other chakras within the body. Use Smokey Quartz, which I feel is the best grounding crystal out there, but just behind that comes Black Obsidian.
Being in the position of the sexual organs, I find Carnelian to have a more calming and soothing effect on one of the most delicate parts of our bodies. Some crystals can have a very strong energy pull, which are best avoided for the sacral chakra.
This chakra is associated with the digestive system which can always play us up a bit, so I tend to use Peridot. They are small but have a strong healing energy, which is released very slowly so not to give your body a shock of focused energy.
Without any doubt the crystal for the heart has to be Rose Quartz. It gives unconditional love on all levels and stimulates the chakra for giving as well as receiving love, especially after a heartbreaking situation.
I feel this is one of the most amazing crystals around. It can give you a clear understanding of the spirit world and is able to link with your spirit guides and guardian angel. Lapis Lazuli really is a must have crystal.