Tourmaline is the best source of

Tourmaline is the one mineral among the given options that is the main source of boron in soils.
Tourmaline is a crystalline silicate mineral group in which boron is compounded with elements such as aluminum, iron, magnesium, sodium, lithium or potassium.
It occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section.