Crystal Watermelon Tourmaline

Chemical Formula

The simple chemical formula, which covers the main forms of Tourmaline (Elbaite, Schorl, and Dravite), is as follows:
(Na,Ca)(Mg,Li,Al,Fe²⁺)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄

The expanded formula, which additionally covers Uvite, Liddicoatite, and Buergerite, is as follows:
(Na,Ca)(Mg,Li,Al,Fe²⁺,Fe³⁺)₃(Al,Mg)₆(BO₃)₃Si₆O₁₈(OH,O,F)₄

The formula for the Tourmaline group is very complex. See The chemical formula of Tourmaline for more details.
 

Composition

See The chemical formula of Tourmaline.

Color

Tourmaline is extremely varied in color. Colors include black, brown, green, red, pink, blue, and gray. White, colorless, yellow, orange, and purple colors are less common. Crystals are frequently multicolored, containing two or more distinct colors. Some specimens are pleochroic.

Streak

White

Hardness

7 - 7.5

Crystal System

Hexagonal

Crystal Forms 
and Aggregates

Usually as elongated prismatic crystals that are heavily striated. Also as short, stubby, prismatic crystals. Most Tourmaline crystals have a rounded, triangular cross-section. Seldom in tabular crystals. Aggregates include columnar, radiating, botryoidal, stalactitic, in dense groups of tiny, elongated needles, and in compact masses.

Transparency

Transparent to opaque

Specific Gravity

2.9 - 3.3

Luster

Vitreous. Some black and brown specimens may be dull.

Cleavage

3,2

Fracture

Conchoidal to uneven

Tenacity

Brittle

Other ID Marks

1) Strongly pyroelectric.
2) Piezoelectric.
3) A few forms of Tourmaline fluoresce yellow in shortwave ultraviolet light.
 

In Group

Silicates; Cyclosilicates; Tourmaline Group

Striking Features

Color, crystal form, hardness, and deep vertical striations.

Environment

Elbaite, Schorl, and Liddicoatite are almost exclusively from granite pegmatites, while Dravite and Uviteor mostly from metamorphic environments such as marbles. Buergerite is from igneous rhyolite deposits.
 

Rock Type

Igneous, Metamorphic

Popularity (1-4)

1

Prevalence (1-3)

2

Demand (1-3)

1

The variety list below shows the main Tourmaline group members, as well as the popular Elbaite variety forms. Please see individual member page for additional variety names.
 

Achroite 

Colorless variety of Elbaite Tourmaline.

Buergerite 

Rare individual member mineral of the Tourmaline group, occurring almost exclusively at Mexquitic, San Luis Potosí, Mexico.

Dravite 

Individual member mineral of the Tourmaline group. It is usually brown in color, and the term may be corrupted to include all forms of brown Tourmaline.
 

Elbaite 

The most well-known individual member mineral of the Tourmaline group. Elbaite is the most transparent and colorful form of Tourmaline. The term Elbaite may be corrupted in the gemstone industry to refer specifically to green Tourmaline.
 

Indicolite 

Blue variety of Elbaite Tourmaline.
 

Liddicoatite 

Uncommon member mineral of the Tourmaline group found primarily in Madagascar. It is the calcium analogue of Elbaite, containing calcium in its chemical formula instead of sodium.
 

Rubellite 

Pink to red variety of Elbaite Tourmaline.
 

Schorl 

Individual member mineral of the Tourmaline group. It is black in color, and the term may be corrupted to include any very dark Tourmaline forms.
 

Uvite 

Uncommon member mineral of the Tourmaline group, usually found in metamorphic environments such asmarbles.
 

Watermelon Tourmaline 

-  Variety of Elbaite Tourmaline that is green on the outside and red on the inside.
 

See the individual Tourmaline mineral pages for detailed locality information for each form of Tourmaline.