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Natural Rough Rubies

Published

A GUIDE TO SEPARATING

Natural Rough Rubies

FROM SYNTHETICS AND SIMULANTS

A common problem when purchasing rough rubies is to be sure that: 

1. The stone is actually ruby (red corundum), 

2. That it is natural (not synthetic/lab-grown) and 

3. That it is gem-quality.

Almost all natural rubies are heat-treated to improve their color and this is an accepted practice in the trade. However, many natural rubies are heated with additional chemical additives that alter their color, clarity and/or transparency – this can have a significant effect on their value. It is not impossible to find untreated rubies in the market – just fairly rare. It is important to note that both natural and synthetic rubies are still ruby (red corundum), they have the same physical, optical and chemical properties. They can only be assuredly separated by a trained gemologist.
Many rubies are treated in their rough form and there have been many cases of mine owners selling buyers synthetic rubies that have been carved and had dirt added to them to fraudulently present them as natural.
A very common gemstone that is found all over the world and is often mistaken for natural ruby is natural red garnet. To those who are not gemmologists, a red stone can definitely look like a ruby but with the correct knowledge and tools, one can easily separate the two.
When a buyer is out at a mine, the tools available are limited, and testing rough gemstones in general – even at a laboratory – can still be challenging.
Below are some tips to help buyers of rough rubies ensure that they are not being fooled. Please note that these tips are only advice and serve as a guide. To be sure of a gem’s identity and quality, it needs to be taken to a capable gemological laboratory. Only tips for separating natural ruby, synthetic ruby and, red garnet are given because they are the most common and therefore relevant; other options are still possible (spinel, glass, tourmaline, etc.). Treatment identification is very complicated and needs a gemological laboratory.

FLUORESCENCE

Fluorescence is the emission of light by a gem that has absorbed energy from a source. In gemology, the two main sources are
in the Ultraviolet (UV) range and are termed long-wave (365nm) and short-wave (254nm). In the field, UV light is very effective at observing fluorescence of gems and is usually long-wave (LW).
The fluorescence of ruby is caused by trace impurities of chromium in its structure; and this is also what is largely responsible for its red color. Another significant trace element in ruby is iron. Iron quenches fluorescence and the iron content of a ruby depends on its growth environment.
Natural rubies almost always fluoresce the color red. The strength of the fluorescence can vary from very strong to none at all.
In general, synthetic rubies have a very low iron content (they are grown this way in a laboratory) – therefore they will almost always have very strong red fluorescence.
Red garnets almost always do not fluoresce.
By using a long-wave UV light source to observe fluorescence, it can indicate the possible identity of the stone and whether it is probably natural or synthetic:

LWUV of rough red stone

None

Faint/Moderate

Strong

Indicates

Garnet or Natural (High Iron) Ruby

Natural or Synthetic Ruby

Natural (Low Iron) or Synthetic Ruby

POLARISCOPE

Made up of a light source and two crossed polarizing filters. They are available as desktop models and various portable models (folding, torch attachment, etc.). The polariscope allows one to determine whether a gem is doubly refractive (DR) or singly refractive (SR). Some gems – such as red garnet – often display ‘anomalous double refraction’ (ADR).

By placing the gem in between the two filters, with the light on, and rotating the stone 360° and carefully observing –
you can determine whether the stone displays DR, SR or ADR.
While rotating the stone, pay attention to any light or dark areas.
ADR stone will ‘blink’ (go from light to dark) during the rotation.
ASR stone will stay dark during the rotation.
An ADR stone will mimic the blinking of a DR stone but actually just has a light background with dark ‘strain’ lines throughout the stone (the pattern of the lines depends on the identity of the stone) – for garnet it will usually look like many fine striations across the stone.

Polariscope of rough red stone

Indicates

Natural or Synthetic Ruby

 

 

 

Garnet

 

 

Garnet

LOUPE

A loupe is a magnification tool that allows the user to see finer details of a gem and sometimes inside the gem itself – depending on the magnification level. The standard is 10x but there are loupes with higher magnification.
By using a loupe, one can assess a gem and with the correct knowledge and experience, one can possibly use the observations to identify the gem (a microscope is much better) but only with the help from other tools – like those mentioned above.
When assessing potential rough ruby, the first thing one can look at is the morphology of the crystal. Not all rough stones will be perfect crystallographic specimens but by looking at the symmetry, angles and crystal faces, one can gather useful information as to the gems crystal system, crystal habit and possible identity.
Garnets are part of the cubic crystal system and are commonly found as dodecahedrons and trapezohedrons.
Ruby (Corundum) is part of the hexagonal crystal system and is commonly found as prismatic hexagonal crystals or waterworn pebbles.
Another important thing to look at is the inclusions and growth features inside the stone. Learning what inclusions and growth features are found in natural and synthetic ruby is very important in order to separate the two. In general, if a stone has no inclusions in it, even under high magnification, it should be suspected as synthetic.
These three tools, as well as the use of others such as a spectroscope and dichroscope can be very useful in identifying natural rough ruby in the field.

IMPORTANT: Final judgments should always be left to trained gemologists with access to the necessary gemological equipment. The tools mentioned above are useful for obvious cases.
There are many cases where rough synthetic ruby has been fashioned to look like natural ruby and sold as such – even by mine owners.