Tourmaline- Colorful by Nature

Chemical Composition and Structure of Tourmaline
 
Tourmaline (Ca,K,Na)(Al,Fe,Li,Mg,Mn)3(Al,Cr, Fe,V)6 (BO3)3(Si,Al,B)6O18(OH,F)4
is a crystalline boron silicate mineral compounded with elements such as aluminum, iron, magnesium, sodium, lithium, or potassium. Classified as a semi-precious stone and gemstone, tourmalines are minerals with an incomparable variety of colors. The name tourmaline is derived the Tamil and Sinhalese word "Turmali", which applied to different gemstones found in Sri Lanka. In translation, the word is thought to mean 'stone with mixed colors', referring to the color spectrum of this gemstone, which outdoes that of all other precious stones. There are tourmalines with colors ranging from blue to yellow and from red to green.
Formula: X(Y3)Z6(T6O18)(BO3)3V3W
X = Ca, Na, K, or is vacant (large cations);
Y = Al, Fe2+, Fe3+, Li, Mg2+, Mn2+
Z = Al, Cr3+, Fe3+, V3+ (small cations);
T = Si (and sometimes minor Al, B3+);
V = O and/or OH;
W = F, O and/or OH.
 
Tourmaline is a ring cyclosilicate containing a six-member trigonal crystal system. With no other common mineral having three sides, tourmaline is notable for its three-sided prisms. It is further differentiated from other minerals because its prisms faces often have heavy vertical striations that produce a rounded triangular effect.
With long slender to thick prismatic and columnar crystals that are usually triangular in cross-section, tourmaline is often found with curved striated faces. The crystal is hemimorphic because the termination at the ends of crystals is sometimes asymmetrical. Such hemimorphic crystals form radial daisy-like patterns that are common in a fine-grained granite called aplite. All hemimorphic crystals are piezoelectric and generate electric charges in response to applied mechanical stress. Most hemimorphic crystals are also pyroelectric or capable of generating an electrical potential when cooled or heated.
The compositional differences and unique structure of tourmaline make it a mineral known for exhibiting a variety of colors. In most cases, magnesium-rich varieties of tourmalines are brown to yellow, iron-rich ones are black to bluish-black to deep brown, and lithium-rich tourmalines are almost any color: blue, red, green, pink, yellow, etc.  Bi-colored and multicolored tourmaline crystals are common, demonstrating variations of fluid chemistry during crystallization. A notable example of this property is in “Watermelon Tourmaline” or a tourmaline crystal that is pink at one end and green at the other. Some forms of tourmaline are dichroic, meaning they change color when observed from different directions.
Fluor-liddicoatite
An example of a multicolored tourmaline is Fluor-liddicoatite. It is a is a rare member of the tourmaline group of minerals, with a formula of Ca(Li2Al)Al6(BO3)3Si6O18(OH)3F. Its properties are defined in the table below:
Chemical Composition and Structure of Tourmaline