Tourmaline is a crystalline boron silicate mineral. The specimen is composed of black tourmaline in massive granular form, set in granite.The structure of the crystals is not immediately obvious because of the massive nature. The inset central photograph is approx 5 mm x 5 mm and is taken from the surface of the smaller specimen. The parallel acicular crystals can be seen, but even the longest crystals are only 2 mm in length. The specimens location is only known as Cornwall.