Ruby Sources

Christopher P Smith, C R ‘Cap’ Beesley, Elizabeth Quinn Darenius, Wendi M Mayerson writes:

Deposits of gem quality ruby may be found in approximately 20 countries around the world. However, most are concentrated in two regions where major geologic events have occurred. The first involves the orogeny or mountain-building event that is responsible for the creation of the Himalayan mountain chain—and several other associated mountain ranges—that occurred when the Eurasian and Indian plates collided approximately 55 million years ago. These ruby deposits extend from the remote areas of Afghanistan, Pakistan and Tajikistan, through Nepal into Burma and across to Vietnam.

The second major geologic event that is responsible for a number of ruby deposits, as well as other gems, includes the Pan-African orogeny, estimated to have taken place approximately 800 million to 450 million years ago. This mountain-building process occurred prior to the separation of a large landmass known as Gondawana, when what are presently Eastern Africa, Sri Lanka, Madagascar and India were are all united. This event is responsible for the ruby deposits of Southern India, Sri Lanka, Madagascar, Kenya, Tanzania and Malawi.

A number of other ruby sources are associated with eruptive events at the continental margin of the Pacific plate’s western edge. Along this subduction zone, erupting magmatic rocks have intersected ruby occurrences that formed deep in the mantle and transported them to the surface. These ruby sources include Thailand, Cambodia, Laos and Australia. Other magmatic-related deposits that occasionally produce rubies include Kenya, Colombia and the U.S.

Random occurrences of ruby have also been found in the Ural mountains of Russia, Greenland and Brazil, as well as elsewhere.

The Growth of Ruby
All known ruby deposits are formed under metamorphic growth conditions. Metamorphism is when preexisting rocks are recrystallized without melting due to changes in temperature, pressure and/or chemical composition on a regional or local scale. A number of specific metamorphic environments are responsible for the growth of rubies within individual countries or areas. Worldwide, gems may be recovered from primary or secondary deposits, that is within the rocks where they formed or a secondary location where they had been transported, respectively. Not considering transportation, gem quality ruby deposits can be categorized into two main groups: metamorphic and magmatic-related.

Metamorphic deposits: This broad grouping of corundum deposits includes a range of specific metamorphic environments, such as marbles, mafic and ultramafic rocks, as well as those characterized by high fluid rock interaction and metasomatism of different preexisting rocks—desilicated pegmatites in mafic and ultramafic rocks, skarns, etc. However, the most important of these in terms of producing gem-quality rubies are those ruby deposits occurring in marbles.

Magmatic-related deposits: These ruby deposits required magmatic, eruptive events to transport the rubies to the surface. However, the magmatic rock—alkali-basalt, lamprophyre and syenite—is not where the rubies formed. Although these rubies have a metamorphic origin, they have a unique set of gemological characteristics that clearly distinguish them from the rubies recovered from the metamorphic deposits described previously.

Source Type Classification
A system of classification was developed by of the authors, C.P. Smith—while director of the Gubelin Gem Lab in Switzerland—that combines geologic and gemological considerations in order to compare rubies with similar properties and internal characteristics forming in various growth environments. This system allows for rubies of similar ‘types’ but from different countries to be compared, as well as rubies of different ‘types’ to be distinguished from each other. This classification can also be applied to sapphires.

The system has two tiers. The first tier separates rubies into three groups based on broad geologic formation scenarios. Two of the three groups contain stones that possess what are considered by experienced gemologists to be ‘classical’ combinations of specific gemological features for metamorphic and magmatic-related sources. For the rubies from metamorphic environments, referred to as Met, such features include a bright red color and relatively low iron content. Typical sources for the Met group include Burma, both Mogok and Mong Hsu, Vietnam, Luc Yen, and Tajikistan. Rubies from magmatic-related environments, referred to as Mag, have classical features that include a dark red, often purplish color, a relatively higher iron content and very specific inclusions. Typical sources of ruby in the Mag group include Thailand and Cambodia. A third group, designated as Met-Mag, signifies those sources that have properties and characteristics outside of the classical ones for either a metamorphic or magmatic-related deposit. However, they are characterized by a body color that has modifiers of brown and/or orange and some of the highest iron contents of any ruby sources. Ruby deposits that fall into the Met-Mag group include Malawi, Tanzania, both Songea and Winza, and Madagascar.

The second tier of the classification system subdivides each of these three groups into four categories of ‘types’, Type I-IV, based on their dominant inclusion features and supported by various other features and data from advanced analytical techniques. Type I stones are typified by ‘silk’ inclusions, most commonly needles of rutile. Type II are characterized by very fine-grained, zonal clouds of ‘dust’ particles. Type III may be distinguished by various crosshatch and flake-like inclusion patterns and Type IV are identified by concentrations of zircon inclusions and/or negative crystals associated with thin films that typically bisect the negative crystals, referred to as ‘equatorial thin films’. Combinations of these types may also occur when multiple features are encountered in a stone.
(Source: Rapaport, Vol 31, No.47, December 5, 2008)

Useful links:
www.aglgemlab.com
www.gubelinlab.com

I found the report in the Rapaport magazine interesting. The color pictures and inclusions are good reference sources for educational purposes, but how practical/applicable are the type categorization/classification in the gem trade. Because of the high value status of rubies and sapphires among the traders/consumers/collectors a slight variation in the hue/differences in opinion among experts could cost thousands of dollars. High tech treatments of rubies and sapphires have become a common practice. Detecting treatments/origin have also become difficult. But I believe it's a small step in the right direction. Hats off to the AGL Gemlab team.