| Gemstone |
Crystal
System |
Density |
Hardness |
Refractive
index |
Treatments |
| Corundum
|
trigonal |
4.00 |
9.0 |
1.762-1.770 |
heat,
fracture filling;
bulk diffusion with beryllium, coating, irradiation |
|
| 
Color:
Every color of the rainbow
Clarity:
Transparent
to Opaque;
Type II
Optic Character: DR
Uniaxial Negative
Cause
of Color:
Purple: Fe, Ti Cr in varying combinations
Pink: Cr, possibly Ti
Blue: Ti, Fe
Green:Fe or Fe and Ti
Yellow: Fe
Orange: Fe or Color Centers
Orangey Pink: Cr
Color Change: Cr, V, Fe, Ti
Fluorescence:
Violet and Colr Change::Inert to Moderate to Strong Red
Pink: Strong Orange-res
Blue: Inert to Strong Red; dependent on Fe content
Green:Inert
Yellow: Inert to moderate orange, red or yellow
Orange: usually inert
Orangey Pink: May be strong orange-red
Fracture
:
Concoidal
Luster
:
Vitreous
Cleavage : None
Durability:
Excellent
Characteristics
for Identification:
Silk
(rutile or boehmite), zircon crystals (sometimes with halos),
hexagonal growth lines. fingerprint inclusions, hexagonal
growth lines, color zoning
Localities:
Madagascar, Burma, East Africa, United States, Australia
|
Corundum
Al2O3
(aluminum oxide)
Corundum
is an aluminum oxide that occurs in every color of the rainbow.
When it is red it is termed a ruby.
When it occurs in any other color it is termed a sapphire.
There are two primary ways that corundum is formed. One
is the metamorphosis of limestone and the other is an igneous
occurrence in rocks lacking in silica. Since corundum is
so hard it is very resistant to weathering. Therefore, it
accumulates in placer gravels. Placer deposits in Sri Lanka
at Ratnapura have been mined since before the time of Buddha.
The major sources today for rubies are Burma and Madagascar.
The major source for sapphire is Madagascar. Other locals
include Pakistan, Afghanistan, India, East Africa and Yogo
Gulch in Montana.
Corundum
can be confused with:
Chrysoberyl
Garnet
Spinel (Natural and Synthetic)
|
Predicted Results of Heat
Treatment on Corundum*
Process |
Result |
| FRACTURE FILLING |
Surface-reaching
fissures/cracks and cavities may be filled with additives
to improve their clarity, luster and appearance |
| DEVELOP ORANGE
|
Dark/purplish
red rubies may turm orange to golden-orange, specially when
beryllium is used as additive in the process |
| INTENSIFY YELLOW,
DEVELOP ORANGE |
Pale
yellow sapphires may be intensified to deeper yellow, or may
turn orange/yellow and other colors. When beryllium and other
additives are used the color improvment is dramatic. |
| DEVELOP BLUE |
Milky-silky (geuda)
or partially blue ("ottu", "ural") and
other types of corundums may turn to near-uniform blue. |
| SURFACE/SUBSURFACE
COLOR DIFFUSION |
Lab prepared additives
heated with selected natural, synthetic, or previously treated
corundums may produce near-uniform blue, pink, orange, orange-pink
coloration. The color is confined at/near the surface/subsurface
of the stone. |
* The
Heat Treatment of Ruby & Sapphire
by Ted Themelis
|
|
The
following books are my recommendations for further study.
They are some of the best available.
If you would like a larger selection, please visit the Book
Shop.
|
|
