PRECIOUS STONES

STONES

Lapis lazuli
Turquoise
Malachite and Azurite

(1) Lapis Lazuli

Lapis Lazuli is normally a mixture of three minerals:

Lazurite (very complex blue mineral)
calcite (calcium carbonate, which is white)
pyrite (an iron sulfide that is white-gold in color)

lazurite is the essential ingredient of lapis lazuli and is the mineral that gives it the blue color. The best quality material contains less calcite and pyrite

lazurite is a sodium, calcium, aluminosilicate mineral that contains sulfur: the color is due to a charge transfer between sulfur atoms

it is a felspathoid (somewhat feldspar-like)
cubic (thus, isotropic)
hardness: 5.5
S.G. 2.7 - 2.9
vitreous / greasy luster

See a movie on a lapis (sodalite) structure!

Comments

The name lapis is Persian for blue.

Prior to 1828, it was used as pigment for ultramarine paint.

For more that 6000 years, it has also been used as an ancient precious stone/gem in Afghanistan, and traded throughout Africa and Europe.

Formation of lapis lazuli

Lapis lazuli, is a contact metamorphic rock with variable composition and varying physical properties. It usually forms by contact metamorphism of limestones.

Uses of lapis, durability, and simulants

Use of lapis lazuli: beads, cabochons, carvings

"Durability": sensitive to heat, acid, alkali

sometimes dyed or coated with wax
Swiss Lapis is dyed quartz

Other comments

Copper-bearing minerals formed at low temperatures: turquoise, malacite, and azurite

Turquoise commonly occurs in veinlets penetrating weathered, aluminum - rich, sedimentary or volcanic rocks (where it has been deposited near the surface from circulating phosphatic waters) in arid climates, and in small, fine-grained, rounded masses and crusts.

Formation of turquoise, malachite, and azurite

(a) Turquoise:

turquoise is a "cryptocrystalline" material: it's made of tiny crystals (you will rarely see individual crystals)
it's a hydrous (water) copper (Cu) - bearing phosphate mineral
hardness= 5 - 6
color: medium green - blue (greener material contains more Fe)
greasy (waxy) luster
other information

Turquoise is formed in association with Cu deposits; copper is transported in waters which interact with phosphorous and aluminum-bearing rocks.
Turquoise is often veined and frequently contains inclusions of surrounding rocks.
sometimes occurs as kidney-shaped or grape-like aggregates.

Geographical: well known deposits occur in Persia, Sinai Peninsula, China, Chile, Egypt, Turkey, Mesoamerica (Arizona)

Other name: Turkish stone

Turquoise's use dates back to 3000 B.C.- 4000 B.C.+ (Sinai)

pins

Stability: sensitive to temperature: above 250 degrees C, a loss of water leads to a dull green color

Enhancements: impregnated with plastic, parafin, oil

Synthetics: ceramic material containing small blue spheres

Simulants: glass, plastic, dyed material (e.g., calcite)

imitations can be found from 4000 B.C.!

(3) Malachite and Azurite

(b) Malachite:

copper (Cu) - carbonate mineral

soft, green material
hardness= 3.5 - 4.0
S.G.: 3.75 - 3.95
crystal system: monoclinic
bubbles with acid
it commonly grows as aggregates of crystals that form concentric rings.

Geological: Cu-bearing solutions interact with carbonate rocks (cavelike deposits)

(Ancient) Uses:

jewelry, ornaments, carving, etc.
make up (eye shadow)
green pigment
cure for vomiting
protection from witches!

Cu-bearing carbonate
a soft, blue material
hardness: 3.5 - 4.0
S.G.: 3.7 - 3.9
vitreous luster
refractive index: 1.73 - 1.758 max. 1.838
crystal system: masses of tiny monoclinic crystals
occurrence: Arizona, Chile, Rhodesia

Azurite commonly occurs with malachite (azur-malachite is an intergrowth of the two minerals). Because of its similarity with malachite (formed in similar way), azurite has similar physical properties and uses.

View some images of malachite, azurite, and turquoise

Pyroxenes (and Amphiboles), Tourmaline and Garnets

Precious Stones: Agate and Opal

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