PHENOMENA

K. Phenomena

1. Unusual optical effects displayed by some gem varieties.

2. Types

a. Caused by reflection of light from inclusions.

1) Chatoyancy (cat's eye)

a) Cause

i) Minute, parallel, needle-like inclusions or hollow tubes.

ii) Cat's-eye band perpendicular to needles.

iii) Moves with light source.

b) To display, cat's-eye stone must be cut in a cabochon.

c) Examples

i) Varieties of chrysoberyl, beryl, quartz, feldspar, tourmaline.

ii) Not tiger's eye quartz (pseudomorph after crocidolite).

d) See Fig 6.6, page 77, in text.

2) Asterism (stars)

a) Cause

i) Minute, oriented, needle-like inclusions.

ii) Three or more chatoyant bands.

iii) Band perpendicular to needles.

iv) Bands Intersect at 60 degrees.

b) Stone must be cut in a cabochon with the girdle parallel to the needles.

c) Examples

i) Corundum (hexagonal -rhombohedral crystal system with 6-rayed star).

ii) Almandine garnet.

iii) Quartz.

iv) Feldspar.

v) Diopside.

d) See Fig. 6.7, page 77, in text.

i) Feldspar.

ii) Diopside.

3) Aventurescence (glittery)

a) Caused by tiny platelets or flake-like inclusions of another mineral.

b) Examples

i) Sunstone (oligoclase feldspar with hematite flakes, causing a gold shimmer).

ii) Aventurescence quartz (inclusions of fuchsite)

b. Caused by selective absorption.

1) Change of color.

a) Stone selectively absorbs all colors except green or red dependent upon whether the light is incandescent or daylight.

i) Stone greenish in daylight (fluorescent light) which is rich in blue and green wavelengths.

ii) Stone reddish in incandescent light (light bulb) which is mostly of red and red-orange wavelengths.

b) Only true change of color gem is alexandrite (variety of chrysoberyl; any other gems are described as alexandrite-like).

c. Caused by interference and diffraction of light.

1) Play of color.

a) Multiple colors displayed by fine opals.

b) Caused by transparent spheres of amorphous silica and the voids in between.

i) Spheres and voids form a 3-D grating.

ii) Light interference and diffraction produce the colors.

c) Varieties of opal have the additional phenomena of asterism.

2) Labradorescence (flash of color).

a) Caused by repeated twinning of mineral labradorite (feldspar group).

b) Light interference from very thin parallel twinning planes.

3) Iridescence (prismatic colors).

a) Caused by light interference from thin layers of trapped gas or liquid (oil on water effect); of different RI than host.

b) Conchoidal fractures can have iridescence when separation plane of the fracture has trapped gas (air).

c) Examples

i) Iris quartz

ii) Iris agate

iii) Fire agate

iv) Obsidian

4) Adularescence (billowy light).

a) A blue or blue white billowy light.

b) Caused by diffusion of light from:

i) Minute concentrations of another feldspar with slightly different RI or,

ii) Repeated twinned.

iii) Stone must be cut in a cabochon.

iv) Only example is moonstone (orthoclase feldspar).

5) Orient

a) Prismatic sheen.

b) Scale-like structure of calcite and aragonite in the pearl causes interference and diffraction of light.