Spinel - Locality: Hunza Valley, Gilgit District, Northern Areas, Pakistan. Size: 2.6 x 2.2 x 1.9 cm. A striking, 1.6 cm, lustrous, compound, octahedral, indigo-blue spinel crystal set atop contrasting snow-white marble from the Hunza Valley of Pakistan. Pristine, complete all-around and eminently viewable from both sides. By Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0], via Wikimedia Commons
Highest values go to pure blue, size and clarity.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
| Blue | .5 to 2 carats | 2 to 4 carats | 4 carats plus |
| Faceted | to /ct | to /ct | to /ct |
| Red | .2 to 1 carat | 1 to 5 carats |
| Faceted | to /ct | to ,000/ct |
| Pink | .5 to 1 carats | 1 to 3 carats | 3 carats plus |
| Faceted | to /ct | to ,000/ct | to |
| Purple | .5 to 1 carats | 1 to 3 carats | 3 carats plus |
| Faceted | to /ct | to ,800/ct | to ,000/ct |
| Other Colors | .5 to 1 carats | 1 to 5 carats | 5 carats plus |
| Faceted | to /ct | to /ct | to ,000/ct |
| Star Spinel | All sizes |
| to /ct |
| Data | Value |
|---|---|
| Name | Gahnospinel |
| Is a Variety of | Spinel |
| Colors | Green or blue, usually dark in tone |
| Crystallography | Isometric. |
| Refractive Index | 1.716-1.780 |
| Luster | Vitreous. |
| Polish Luster | Vitreous. |
| Fracture Luster | Vitreous. |
| Hardness | 7.5 - 8 |
| Wearability | Good |
| Fracture | Conchoidal |
| Specific Gravity | 3.58-4.41 |
| Birefringence | None. |
| Cleavage | Indistinct |
| Dispersion | 0.020 |
| Luminescence | Inert. |
| Luminescence Present | No |
| Enhancements | None known. |
| Transparency | Transparent to translucent |
| UV Long | Inert |
| UV Short | Inert |
| Absorption Spectrum | Bands at 5700 and 7000, not always present. |
| Phenomena | None. |
| Identifying Characteristics | Dark tone |
| Formula |
(MgZn)Al2O4 |
| Pleochroism |
None. |
Gahnospinels from Sri Lanka
|
Gahnospinels from Sri Lanka |
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|
ZnO(%) |
FeO(%) |
N |
S.G. |
|
0.14 |
2.34 |
1.716 |
3.6 |
|
7.18 |
1.92 |
1.723 |
3.72 |
|
10.27 |
2.52 |
1.731 |
3.77 |
|
12.98 |
1.47 |
1.737 |
3.86 |
|
18.21 |
1.93 |
1.747 |
3.97 |
|
24.81 |
1.90 |
1.752 |
4.05 |
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