How to distinguish spinel from ruby?

Nov 16, 2022

First of all, spinel and ruby are two kinds of gems that are often confused in history. The main reason is that they have very similar appearances, especially in Myanmar, where the two are often associated, so spinel is often mistaken for ruby. The most famous gems mistaken for ruby in history are the following two.

1. Temuer Ruby

Although its name is "Ruby", its real identity is spinel, and the owner of the gem is engraved on the front of the gem. After several changes of ownership, this gem once fell into the hands of the conquerors of the Tatars. In 1398, Tiemuer obtained this gem, and finally named it "Tiemuer Ruby"; In the 17th century, this gem belonged to the British royal family and is now kept in Buckingham Palace in London, England.

2. Black Prince's Ruby

Seeing the crown below, many people who are familiar with diamonds will know better that it is inlaid with the "Kurinan No. 2" cut from the largest diamond raw stone in the world; However, today's focus is not on diamonds, but on spinels. The red gem above is spinel, weighing 170 carats, but its origin is unknown, and its cutting method basically retains the original stone shape. It is currently kept in the British royal family.

However, the history of confusion between the two was basically ended in 1783. After analyzing the chemical composition, the mineralogist Jean Baptiste Louis found that the two gemstones were completely different. However, from the perspective of modern gemology, it was relatively easy to distinguish them.

1、 Difference of raw stone

It can be found from the Chinese name that there is a word "sharp" in the spinel, while the English name "Spinel" comes from the Latin word "Spinella", which is translated as "tip, thorn". It can be inferred from either name that the original stone of the spinel should have relatively sharp edges and corners. Its original stone form is mainly octahedron.

However, spinel occasionally has twin crystals (that is, two or more crystals grow together) and parallel intergrowth (it can be simply understood that two crystals grow together, but all faces/edges are parallel to each other).

Spinel triplet

Spinel parallel intergrowth

The original shape of ruby is very different from that of spinel, which is generally similar to barrel shape. According to the aggregation analysis, it can be found that the shape of ruby is composed of hexagonal columns, hexagonal bipyramids, prisms and parallel double faces. The figure below shows the original ruby, weighing 1700 carats.

2、 Comparison of gemological parameters

If gemstones have been cut and ground into finished products, we can distinguish them by comparing a series of gemological parameters.

1. Different refractive index

Ruby is 1.762-1.770, which is also a birefringent gem, but the refractive index of spinel is about 1.718, which is obviously smaller than ruby, and it is a single refraction gem.

2. Different polychromaticity

Generally, in order to show the best color of ruby, the table top of the finished product is often perpendicular to the waist edge, so when observing ruby along the waist edge, obvious dichroism will be observed, but the spinel is a single refracting gem, without polychrome, so it is easy to distinguish it with a dichroic mirror.

3. Distinguishable under polarizer

Ruby is a tripartite crystal system, so it belongs to optical inhomogeneous body. When it is rotated 360 ° under the polarizer, four bright and four dark phenomena can be observed;

Spinel is an equiaxed crystal system, belonging to optical homogeneity, which should be a whole case or an abnormal extinction phenomenon under polarized light.

4. Different internal characteristics

It is often used to describe the fact that there are many ruby fractures, and the inclusions inside are mainly fingerprint inclusions/mineral inclusions/gas-liquid inclusions/rutile needles;

However, spinel usually has negative crystals of octahedron and often exists in groups.

The left picture shows the octahedral negative crystals in groups inside the spinel, and the right picture shows the fingerprint makeup inclusions in the ruby

The left picture shows the long needle shaped ruby inclusions in Sri Lanka, and the right picture shows the short needle shaped ruby inclusions in Myanmar.

5. Different color bands

Ruby usually has straight color bands, but spinel is rare. The following figure shows the hexagonal color bands in starlight ruby

4、 Special varieties

There is a special variety of rubies called "Dabiz Ruby", which is similar to the starlight ruby with fixed star lines, but it is caused by the regular arrangement of internal inclusions in the world, but spinel does not have this variety.

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How to distinguish spinel from ruby?