Tourmaline is a colorful semi-precious gemstone with a hardness of 7–7.5 on the Mohs scale. Its chemical composition is a complex boron silicate containing aluminum, iron, magnesium, sodium, lithium or potassium. Paraiba tourmaline is the rarest and most expensive variety. In Sri Lanka, less common types include rubellite, verdelite, indicolite, black tourmaline, bi-color tourmaline, watermelon tourmaline, yellow and brown tourmaline. Tourmaline is also one of October’s birthstones. In gemstone beliefs it symbolizes protection, balance, creativity and positive energy, while black tourmaline is believed to repel negativity.