THE MINERAL SPINEL

Chemistry: MgAl2O4 , Magnesium Aluminum Oxide
Class: Oxides
Group: Spinel
Uses: as a gemstone and as a mineral specimen
The physical properties of spinel.
We sell natural spinel Specimens.

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 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 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.

PHYSICAL CHARACTERISTICS:

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.
Cleavage: None
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, dolomite, corundum and garnets.
Notable Occurrences include Burma, Sri Lanka, Brazil and Afghanistan.
Best Field Indicators are twinned crystals if present, color, hardness, density and locality.

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SPINEL specimens:
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SPINEL specimen spi-1
$ 120.00

Dims: 3-1/2" x 2/38" x 2-1/8"

Wt: 8.1 oz

MacDonald Island vicinity, Northwest Territories, Canada

This specimen is composed of delightfully large (up to 3/4 inch diameter), black Spinel crystals in a white crystalline matrix whose identity I can't figure out. These crystals look a lot like blue-black garnets, in a cubic crystal shape that I'm having trouble digesting, as there are 4-, 5-, and 6-sided faces on every crystal. This is a super-neat piece, and the first raw Spinel crystals that I've ever laid eyes on.

spi-1 ($120.00)

MacDonald Island vicinity, Northwest Territories, Canada

SPINEL specimen spi-2
$ 50.00

Dims: 7/8" x 3/4" x 1/2"

Wt: 8.1 oz

Parker Mine, Notre Dame de Laus, Quebec, Canada

This Spinel cluster is made up of 5 discernable crystals, the largest of which is almost 3/4" across its longest axis. All are black and opaque, with what seem to be blue-black highlights. They also all show the classic octahedral crystal shape, and have a dull luster, with some heavy scratching on the crystal faces. Only one of the crystals is complete, whereas the other four are all missing portions. There is no visible host rock. This is one of the better thumbnails I've seen.

spi-2 ($ 50.00)

Parker Mine, Notre Dame de Laus, Quebec, Canada

SPINEL specimen spi-3
$ 45.00

Dims: 0.3" x 0.3" x 0.3"(0.8 x 0.8 x 0.8 cm)

Wt: 3.15 ct

near Mogok, Burma

This specimen consists of a somewhat warped but nearly complete octahedral crystal of Ruby Spinel. Though lacking perfect symmetry and form, there is only one spot of obvious damage along one edge of the crystal. Its faces show considerable growth patterning and its edges are stepped and show considerable wear, but its form is definite. Its color is a deep, ruby-red, and it has a vitreous luster. It is transparent but contains many internal flaws and fractures. I jumped at the chance to buy some of these- they are the only ones that I found at the shows.

spi-3 ($ 45.00)

near Mogok, Burma

SPINEL specimen spi-5
$ 55.00

Dims: 0.4" x 0.3" x 0.3"(1.0 x 0.8 x 0.8 cm)

Wt: 4.45 ct

near Mogok, Burma

A rather heavily damaged Ruby Spinel octahedron constitutes this Burmese specimen. Its octahedral form is slightly flattened, and 3 of its points are damaged, showing conchoidal chipping. Two of the intact points, however, show natural notching, indicating slight Spinel-Law twinning. The faces are generally clean and the edges, though somewhat rounded, are definite. Its color is bright red, it has a vitreous luster, and it is transparent, though many internal fractures and a few veil-like inclusions interfere with its clarity. It does, however, come from a well known Spinel-producing area of the world, and we have had trouble finding such specimens. Besides, its color is splendid!

spi-5 ($ 55.00)

near Mogok, Burma

SPINEL specimen spi-6
$ 34.00

Dims: 1.7 x 1.4 x 0.6" (4.3 x 3.6 x 1.5 cm)

Wt: 19.4 g

Notre Dame du Laus, Quebec, Canada

This small thumbnail specimen consists of a cluster of heavily-intergrown Spinel crystals that are partly embedded in a small amount of pale orange calcite. I count at least 7 different crystals, or attempts at the formation of individual crystals- as most of them are aligned with each other, I suppose that one could refer to the cluster as a single unit. All are in excellent condition, showing no human-induced damage, and together, have dimensions of 0.5 x 0.4 x 0.4" (1.3 x 1.0 x 1.0 cm). Their octahedral form is quite good, considering their intense intergrowth, and all edges are relatively well-defined, and all primary and secondary faces are flat, generally clean, and possess a bright pearly luster. Their color is basic black, and thus they are all opaque.

spi-6 ($ 34.00)

Notre Dame du Laus, Quebec, Canada

SPINEL specimen spi-7
$ 78.00

Dims: 2.8 x 1.9 x 1.6" (7.1 x 4.8 x 4.1 cm)

Wt: 5.05 oz. (143.2 g)

Aliabad, Hunza, Northern Areas, Pakistan

At least 7 Spinel octahedrons rest in the white host rock of this hand specimen. These crystals generally do not exceed 0.1" (3 mm) in diameter, but one is much larger, having a diameter of 0.3" (8 mm). Most are in excellent condition, showing no damage, but one or two are visibly damaged, and one is mostly incomplete. Their octahedral form is excellent- all have well-defined edges and clean faces that possess a rather dull, pearly luster. Their color is a rather dull, deep violet, and only the smaller crystals are visibly transparent. The largest crystal shows patches of shallow transparence, but is moderately to dimly translucent as a whole. They are accompanied by several small and generally incomplete muscovite books that have a pale brown color and are impressively clear. The host rock in which all of these rests is quite calcareous, though I believe that it contains a substantial amount of feldspar.

spi-7 ($ 78.00)

Aliabad, Hunza, Northern Areas, Pakistan

SPINEL specimen spi-8
$ 34.00

Dims: 0.5 x 0.5 x 0.3" (1.3 x 1.3 x 0.8 cm)

Wt: 6.3 g w/ base

Muchara Nala, Hunza, Northern Areas, Pakistan

This small thumbnail specimen consists of at least 3 Spinel octahedrons that are partly embedded in a white pegmatite base. Two of these octahedrons are partly intergrown, whereas the third (and largest) is isolated. The smaller crystals do not exceed 0.1" (3 mm) in diameter, but the larger one measures 0.3 - 0.4" (8 - 10 mm) in diameter. However, this large crystal is only about 60 - 70% complete and shows slight but visible damage, whereas the smaller crystals, apart from their intergrowth, are essentially complete and undamaged. Their octahedral form is generally very good, with well-defined edges and clean faces that possess a pearly luster. All have a dull, deep violet coloration. The larger crystal is generally dimly translucent, showing small areas of transparence, whereas the smaller crystals are dimly transparent under bright light. They are accompanied by a few broken, pseudohexagonal muscovite books. The piece is attached to a small acrylic base with a removable putty.

spi-8 ($ 34.00)

Muchara Nala, Hunza, Northern Areas, Pakistan

SPINEL specimen spi-9
$ 30.00

Dims: 1.7 x 0.7 x 0.5" (4.3 x 1.8 x 1.3 cm)

Wt: 12.4 g

Aliabad, Hunza, Northern Areas, Pakistan

This small Spinel specimen confuses me a bit. It consists of at least 2 Spinel crystals that are rather heavily intergrown. The larger crystal appears to be slightly damaged and not quite complete, though the smaller one does not show any visible damage. The larger crystal has visible dimensions of 0.3 x 0.3 x 0.2" (8 x 8 x 5 mm), whereas the smaller crystal does not exceed 0.1" (3 mm) in any dimension. Their form is quite odd- it certainly is not octahedral, and does not even appear to be cubic- I cannot define it properly! However, they still have well-defined edges and striated but clean faces that possess a bright pearly luster. Both crystals are colored black and are essentially opaque. The Spinels are accompanied by many rounded muscovite "books" that have a golden-brown coloration and a bright pearly luster. All are embedded in a white, crystalline pegmatite host rock.

spi-9 ($ 30.00)

Aliabad, Hunza, Northern Areas, Pakistan

SPINEL specimen spi-10
$ 45.00

Dims: 0.9 x 0.7 x 0.5" (2.3 x 1.8 x 1.3 cm)

Wt: 5.0 g

Muchara Nala, Hunza, Pakistan

At least 3 Spinel crystals rest in the pegmatite host of this thumbnail specimen. Only one of these exceeds 2 mm along any axis- it has visible dimensions of 0.4 x 0.3 x 0.2" (1.0 x 0.8 x 0.5 cm) and looks as if it has been cleaved in one area. All have reasonably good octahedral form, with well-defined edges and slightly striated but clean faces that possess a bright pearly luster. They all have a dull violet color with a hint of red, and are dimly transparent and contain many internal fractures. A very small amount of muscovite is also present, but does not have any appreciable form.

spi-10 ($ 45.00)

Muchara Nala, Hunza, Pakistan

SPINEL specimen spi-11
$ 57.00

Dims: 0.4 x 0.4 x 0.3" (1.0 x 0.8 x 0.8 cm)

Wt: 3.85 ct

near Mogok, Burma

This small thumbnail specimen consists of a single "Ruby Spinel" octahedron. It is in moderately good condition, showing fresh damage on one corner, and has very good octahedral form that is not quite symmetrical along one axis. It has a ruby-red color and a bright pearly luster, and is transparent and moderately clear, containing a few large internal fractures. There is no host rock present.

spi-11 ($ 57.00)

near Mogok, Burma

SPINEL specimen spi-12
$ 30.00

Dims: 2.1 x 1.7 x 0.8" (5.4 x 4.3 x 2.1 cm)

Wt: 1.7 oz. (47 g)

Muchara Nala, Hunza, Northern areas, Pakistan

A single, twinned Spinel crystal is partly embedded in the pegmatite matrix of this hand specimen. It has visible dimensions of 0.5 x 0.4 x 0.2" (1.3 x 0.9 x 0.6 cm) and is in very good condition, showing little damage. Its octahedral form has been twinned according to the Spinel Law, and though its form is otherwise slightly warped, the twinning is obvious. The crystal has a deep violet coloration and a slightly dull vitreous luster, and is dimly translucent. The surrounding pegmatite is loaded with small flecks of muscovite and probably biotite.

spi-12 ($ 30.00)

Muchara Nala, Hunza, Northern areas, Pakistan

SPINEL specimen spi-13
$ 60.00

Dims: 6 x 4 x 3 mm

Wt: 1.2 ct

near Mogok, Burma

This small thumbnail specimen consists of a single Ruby Spinel crystal. The crystal is in very good condition, showing almost no damage, and has an asymmetrical but very well-defined octahedral form. It has a deep, ruby-red color and a vitreous luster, and is transparent and very clear- a few small internal fractures lie near two opposing surfaces, but that is all. There is no base or host rock present.

spi-13 ($ 60.00)

near Mogok, Burma

SPINEL specimen spi-14
$ 168.00

Dims: 3.9 x 2.8 x 1.8" (9.9 x 7.0 x 4.6 cm)

Wt: 11.6 oz. (330 g)

near Mogok, Burma

This cabinet specimen consists of a calcite matrix in which are embedded at least 4 Ruby Spinel crystals. The largest of these crystals reaches 0.2" (6 mm) in diameter and has the best octahedral form. All have the standard deep ruby-red color and a greasy-to-vitreous luster and are transparent and at least moderately clear. There may be more of these crystals embedded in the gray calcite matrix- maybe careful bathing in hydrochloric acid would be the best way to explore.

spi-14 ($168.00)

near Mogok, Burma

SPINEL specimen spi-16
$ 53.00

Dims: 2.4 x 1.5 x 0.7" (6.2 x 3.7 x 1.7 cm)

Wt: 1.2 oz. (34 g)

near Mogok, Burma

A single, tiny Spinel crystal rests on the quartz base of this hand specimen. The crystal does not exceed 0.1" (3 mm) in diameter but is in exceptional condition. Its octahedral form is likewise excellent, showing well-defined edges and extremely clean faces that possess the standard vitreous luster. The crystal is of a variety known as ruby spinel, and thus has the standard ruby-red color. It is also transparent and extremely clear. The surrounding quartz base shows no crystal form.

spi-16 ($ 53.00)

near Mogok, Burma

SPINEL specimen spi-15
$ 55.00

Dims: 1.9 x 1.4 x 0.8" (4.8 x 3.5 x 2.0 cm)

Wt: 0.8 oz. (23 g)

Mogok, Burma

A single, tiny Spinel crystal rests in the quartz matrix of this hand specimen. This crystal does not exceed 0.2" (4 mm) in diameter and is in excellent condition, showing no damage. Its octahedral form is nearly perfect, with sharp edges and very clean faces. It has a deep red color and a somewhat dull vitreous luster, and is translucent, containing many internal fractures.

spi-15 ($ 55.00)

Mogok, Burma

SPINEL specimen spi-17
$ 150.00

Dims:2.5x2.3x1.4" (6.4x5.8x3.6 cm)

Wt: 3.4oz. (96g)

Baffin Island, Nunavut, Canada

This is a most unusual specimen. Excellent Blue spinel crystals are found in a flesh-colored calcite matrix with diopside and phlogopite mica. There are five exposed spinel crystals on this specimen; one of the smaller ones is damaged. The edges of the spinel crystals are unusually rounded, and they show internal shattering. The largest exposed spinel crystal on this specimen measures 0.5" (1.3cm). This is the single largest crystal seen in this lot of specimens. Also, the calcite on this piece fluoresces a dark purple under short wave ultraviolet light.