Green Tourmaline

Green tourmaline, the variety called verdelite, belongs to the tourmaline group, a boron-rich silicate whose crystals grow as long prisms with a distinctive rounded-triangular cross-section and heavy lengthwise striations. Its green comes from iron or chromium, and single crystals often shift colour along their length or from core to rim, which is why tourmaline yields the famous watermelon stones with a pink centre and green skin.

Like all tourmaline it is pyroelectric and piezoelectric, developing an electric charge under heat or pressure. Tourmaline was not clearly distinguished as a species in Europe until the eighteenth century, when rough was brought from Sri Lanka by Dutch traders, so its gem history is comparatively short and much older emerald in historical accounts may sometimes have been green tourmaline. Its modern metaphysical placement with the heart centre follows from the colour and dates from the twentieth century.

Green tourmaline, trade name verdelite, is a useful place to explain why tourmaline behaves unlike other gem materials.

Tourmaline is not a mineral species. It is a group of borosilicates sharing one complex crystal structure into which an unusual number of elements can substitute: sodium, calcium, lithium, iron, magnesium, manganese, aluminium and others. Elbaite, schorl and dravite are the species most often cut as gems, and a stone sold as verdelite is usually elbaite coloured by iron or by chromium and vanadium.

That chemical tolerance produces two effects that no other gem shows to the same degree. The first is the range: tourmaline occurs in every colour, and a single crystal can pass through several of them. Watermelon tourmaline, pink at the core and green at the rim, is a slice across a crystal whose chemistry changed as it grew, so the zoning is a record of the fluid that fed it.

The second is strong pleochroism. A tourmaline crystal absorbs light differently along its length and across it, and the difference is large enough that a green stone can look almost black down the long axis. Cutters orient the stone to avoid that, which is why tourmalines are so often cut as long rectangles: the shape follows the crystal and the optics, not fashion.

The chromium-bearing greens are sometimes marketed as chrome tourmaline and command higher prices. The distinction is a chemical one that requires testing, and the atlas records it as a trade category rather than a separate stone.