Tourmaline’s colors have many different causes. It’s generally agreed that traces of iron, and possibly titanium, induce green and blue colors. Manganese produces reds and pinks, and possibly yellows. Some pink and yellow tourmalines might owe their hues to color centers caused by radiation, which can be natural or laboratory-induced.

Mineral Tourmaline
Chemistry Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)4
Refractive Index 1.624 to 1.644
Birefringence 0.018 to 0.040
Specific Gravity  3.06 (+0.20, -0.06)
Mohs Hardness  7 to 7.5