Agate is Composed of microscopic crystals of Quartz laid down in colorful bands or ribbons. In Feng Shui, Agate is used primarily anywhere to help bring change. Black agate is a grounding stone, and is used to calming and sooth the mind allowing for inward meditation and reflection.
This particular black agate was chemically treated to give it that interesting denim look. They therefore look more navy than black and are complemented nice with a silver pearl.