REFRACTIVE INDEX

REFRACTIVE INDEX

The refractive index expresses the ratio between the speed of light in air and speed through the stone.

Example:

 Speed of light in air (V1): 300.000 km / sec

 Speed of light in an aquamarine (V2): 190.000 km / sec

 Refractive index of the aquamarine: V1/V2 = 1.58

Each stone has its own refractive index. It has a diagnostic value: it can identify a gemstone. The measurement of this index can be done using a "refract meter" optical instrument convenient and accurate, and the method of immersion, which involves placing a stone in a liquid which is known the refractive index. If the index of the liquid is identical to that of the stone, the outlines of the stone will become invisible, like the ice is when immersed in water.

Refractive index of gems and minerals

Gem / Mineral	Index
Agate	1544-1553
Alexandrite	1745-1759
Almandine	1.76-1.83
Amazonite	1.52
Amber	1.54
Amethyst	1544-1553
Andalusite	1641-1648
Andradite	1.82-1.89
Apatite	1.63-1.640
Aquamarine	1577-1583
Aventurine	1544-1533
Azurite	1.73-1.838
Beryl	1.577-1.60
Brazilianite	1603-1623
Calcite	1486-1658
Chalcedony	1.53-1.539
Chrysoberyl	1.74
Chrysocolla	1.50
Chrysoprase	1534
Citrine	1.55
Cordierite	1.54
Corundum	1766
Demantoid	1.88
Diamond	2417-2419
Diopside	1.68-1.71
Dolomite	1503-1682
Dumortierite	1686-1723
Emerald	1576-1582
Euclase	1652-1672
Fluorite	1434
Gaylussite	1517
Grossular	1738-1745
Hauynite	1502
Hematite	2.94-3.22
Hessonite	1745
Iolite	1548
Ivory	1.54
Jadeite	1.66-1.68
Kunzite	1.655-1.68
Kyanite	1715-1732
Labradorite	1565
Lapis	1.50
Lazulite	1615-1645
Leucite	1.5085
Malachite	1655-1909
Nephritis	1.60-1.63
Obsidian	1.50
Oligoclase	1539-1547
Olivine	1.67
Onyx	1486-1658
Opal	1.45
Orthoclase	1525
Periclase	1.74
Peridot	1.65-1.69
Pyrope	1.74
Quartz	1.55
Rhodisite	1.69
Rhodochrosite	1.60-1.82
Rhodolite	1.76
Rhodonite	1.73-1.74
Ruby	1.76-1.77
Rutile	2.61-2.90
Sapphire	1.76-1.77
Scapolite	1.54-1.56
Serpentine	1.56-1.57
Sodalite	1483
Spessartite	1.81
Sphene	1.885-2.05
Spine	l1712-1736
Spodumene	1.65-1.68
Tanzanite	1.691-1.70
Thomsonite	1.53
Topaz	1.62-1.627
Tourmaline	1624-1644
Turquoise	1.61-1.65
Uvarovite	1.87
Variscite	1.55-1.59
YAG	1.83
Zircon	1.90-2.01
Zoisite	1695

Some liquids used for the method of immersion

Liquid / Gem	Refractive index
Turpentine / Opal	1.47
Oil minerals / Sodalite	1.48
Benzene / Obsidian	1.50
Cedar oil / Petalite	1.51
Oil of cloves / Iolite	1.54
Nitrobenzene / Quartz	1.55
Bromophorme / Emerald	1.59
Carbon disulfide / Tourmaline	1.63
Monobromonaphtalène / Spodumene	1.66

DISPERSION

DISPERSION

The dispersion is the PROPERTY of a stone to break the white light that penetrates in the colors of the spectrum. This property is what causes the rainbow colors of the sky (the "lights" of a stone) is observed especially in Pierre colorless or those with high dispersion index: diamonds, zircon, dermatoid garnet, sphere, and sphalerite are the best examples.

Dispersion index of gems and minerals

Gem / Mineral	Index
almandine	024
andradite	057
Bénitoite	044
Beryl	014
beryllonite	010
cassiterite	071
chrysobery	l015
corundum	018
Danburite	016
Demantoid	057
Diamond	044
dioptase	022
Epidote	030
Euclase	016
Fluorite	007
grossular	028
Idiocrase	019
Peridot	020
pyrope	027
quartz	013
rhodolite	026
scapolite	017
Spessartite	027
Sphalerite	156
Sphene	051
Spinel	020
spodumene	017
glass	010
topaz	014
tourmaline	017
zircon	038

Pleochroism - The color of some gems changes

Pleochroism

The color of some gems changes with the angle at which you look. Diachronic when it comes to these gems have two colors in two directions located at 90 degrees from each other and when trichroisme have three colors in three directions, also located at 90 degrees from each other. The intensity of the pleochroism of a gem is different to the other; it can be high, medium, or low.

Pleochroism stone

Blue
Aquamarine (medium)	colorless-light blue / light blue-dark blu
Alexandrite (strong)	rouge- purple /orange /vert dark
Apatite (strong)	bleu-jaune /bleu-incolore
Benitoïte (strong)	Clear / Dark Blue
Iolite (strong)	Clear / yellow / blue / dark blue-violet
Sapphire (strong)	violet-dark blue / green-light blue
Topaz (very low)	colorless / pale blue / pink
Tourmaline (strong)	dark blue / light blue
Zoisite (strong)	blue / red purple / yellow green
Zircon (strong)	blue / clear / gray

         

Green
Alexandrite (strong)	dark red / orange / green
Andalusite (strong)	dark green / dark red
Emerald (strong)	Green / Blue Green
Peridot (low)	yellow-green / green / colorless
Sapphire (strong)	green / yellow green
Sphene (medium)	dark green / blue green
Tourmaline (strong)
Zircon (low)

                 

Yellow
Citrine (very low)	pale yellow / pale yellow
Chrysoberyl (very low)	rouge-jaune/jaune-vert/vert
Danburite (low)	very pale yellow / pale yellow
Orthoclase (low)	pale yellow / pale yellow
Phénakite (medium)	Clear yellow / orange
Sapphire (low)	yellow / pale yellow
Spodumene (medium)	pale yellow / pale yellow
Topaz (medium)	yellow-brown / yellow / yellow orange
Tourmaline (medium)	pale yellow / dark yellow
Zircon (low)	yellow-brown / yellow

  

Red and pink
Alexandrite (strong) -	dark red / orange / green
Andalusite (strong)	dark red / brown red
Morganite (medium)	pale red / red violet
Ruby (strong)	violet red / orange red
Tourmaline (strong)	dark red / light red
Zircon (medium)	purple / red brown

  

Purple and Violet
Amethyst (very low)	purple / purple
Andalusite (strong)	green brown / dark red / purple
Beryl (medium)	purple / colorless
Corundum (high)	purple / orange
Spodumene (Kunzite) (strong)	Purple / Purple / Clear / Pink
Tourmaline (strong)	pale purple / purple

           

Brown and Orange
Quartz (low	brown / reddish brown
Sapphire (strong)	yellow-brown / orange
Topaz (medium)	brun-jaune/brun-jaune dull
Tourmaline (very low)	dark brown / light brown
Zircon (very low)	brun-rouge/brun-jaune