Page:A treatise on diamonds and precious stones including their history Natural and commercial.djvu/184

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148
APPENDIX.

TABLE of the Specific Gravity, Primitive Form, Electricity, and Refraction of Gems.

Arranged according to their relative hardness.

NAMES OF GEMS. SP. GR. PRIMITIVE FORM. ELECTRICITY BY FRICTION. REFRACTION.
Diamond 3.5 Octahedron Feebly, 10 minutes Slightly double
Sapphire 4.to4.2. Slightly acute Rhomb Strongly[1] Idem
Chrysobyrel 3.8. Parallelopipedon Feebly Idem
Spinelle[2] 3.7 Octahedron Feebly Single
Emerald 2.8 Hexagonal Prism Feebly Double
Topaz 3.5 Rhomboidal Prism Very strongly[3] Double
Aquamarine 2.6 Hexagonal Prism Feebly Slightly double
Hyacinth & Zircon 4.4 Parallelopipedon Strongly Double,in a high degree
Tourmaline 3.2 Obtuse Rhomb Strongly Double
Garnet 4 to 4.4 Rhombl. Dodecaedron Very strongly[4] Single
Rock Crystal, Amethyst Yellow Crystal, &c. 2.5 to 2.7 Obtuse Rhomb Feebly Slightly double
Opal 2 to 2.3      
Peridot 3.3. Parallelpipedon Feebly Double
Turquois 2.4      
  1. ↑ Particularly the white variety.
  2. ↑ The Spinelle and Chrysberyl are equal in hardness.
  3. ↑ Particularly the white variety from Brazil, which will continue many hours.
  4. ↑ Sometimes attract the magnetic needle.
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