Description:
Spinel is a very attractive and historically important
gemstone mineral. Its typical red color, although pinker,
rivals the color of ruby. In fact, many rubies, of notable
fame belonging to crown jewel collections, were found
to actually be spinels. Perhaps the greatest mistake
is the Black Prince's Ruby set in the British Imperial
State Crown. Whether these mistakes were accidents or
clever substitutions of precious rubies for the less
valuable spinels by risk taking jewelers, history is
unclear. The misidentification is meaningless in terms
of the value of these gems for even spinel carries a
considerable amount of worth and these stones are priceless
based on their history, let alone their carat weight
and pedigree. Today, expensive rubies are still substituted
for by spinel in much the same way a diamond is substituted
by cubic zirconia. Not to commit a fraud or theft but
to prevent one. Spinel may take the place of a ruby
that would have been displayed in public by an owner
who is insecure about the rubies safety. The spinel
probably is still valuable but better to lose a $100,000
dollar spinel than a $1 million dollar ruby! Spinel
and ruby are chemically similar. Spinel is magnesium
aluminum oxide and ruby is aluminum oxide. This is probably
why the two are similar in a few properties. Not suprisingly,
the red coloring agent in both gems is the same element,
chromium. Spinel and Ruby also have similar luster (refractive
index), density and hardness. Although ruby is considerably
harder (9) than spinel, spinel's hardness (7.5 - 8)
still makes it one of the hardest minerals in nature.
Spinel may be the poorer cousin of ruby, but its pinker
color and other qualities make it attractive in its
own right. Spinel typically forms in well formed octahedrons.
But it is famous for a type of twinning that bears its
name, the Spinel Twin Law. Spinel Law twinning is also
found in other isometric minerals such as diamond, galena,
cristobalite, magnetite, franklinite and other members
of the spinel group. This type of twinning produces
a twin plane that is parallel to one of the octahedral
faces. The plane acts as a mirror plane and produces
a left and right side that are mirror images of each
other. This may not sound all that spectacular for a
very symmetrical mineral like spinel which is loaded
with mirror planes. However this mirror plane is not
parallel to any of the others and actually lowers the
symmetry of the crystal (only in appearances though).
A good description of the twin is hard to explain, but
here it goes. The plane falls (of course) in the center
of the crystal, dividing it in half. The two octahedron
faces parallel to the twin plane are equilateral triangles.
Each point of the triangles is doubled across the twin plane with an indentation between them. The crystal
looks like it has trigonal symmetry, but the three indentations
are a clue that this crystal is a twin. Twins of spinel
are rare, but their popularity makes them readily available
on the market. |
