THE MINERAL SPINEL
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
Whether these mistakes were accidents or clever
substitutions of precious rubies for the less valuable spinels
by risk taking jewelers, history is unclear.
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.
take the place of a ruby that would have been displayed in public
by an owner who is insecure about the rubies safety.
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
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 nicely proportioned 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
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.
Color is red, green, blue, purple, brown, and black; but red is by far the more common color.
Luster is vitreous.
Transparency: Crystals are transparent to translucent
and sometimes nearly opaque.
Crystal System: Isometric; 4/m bar 3 2/m
Crystal Habits include the typical octahedron, but
can be found as dodecahedrons and combinations of other isometric
forms. Also as rounded grains in alluvial placer deposits.
Fracture is conchoidal.
Hardness is 7.5-8.0
Specific Gravity is 3.6-4.0
Streak is white.
Other Characteristics: index of refraction is approximately
1.71 - 1.76 and rutile
inclusions may produce six or four rayed stars or asterisms.
Associated Minerals include calcite,
Notable Occurrences include Burma, Sri Lanka, Brazil
Best Field Indicators are twinned crystals if present,
color, hardness, density and locality.