Rhodolite - a very special garnet “variety”

During the late 19th century, a remarkable discovery of bright pink to pale violet garnets in Macon County, North Carolina, USA, caused a stir amongst gemologists and mineralogists. These experts quickly identified that these particular garnets possessed a unique composition of approximately one-third almandine and two-thirds pyrope content, making them distinct from previously known pyropes, particularly from Bohemia, and almandines from various deposits such as the European Alps or India. The unprecedented colour and composition of these garnets prompted the suggestion of a new variety name, RHODOLITE, derived from the Greek words "rhodos" meaning rose and "lithos" meaning stone. The name RHODOLITE has since been established as the trade name for this extraordinary gemstone.

Rhodolites are highly prized for their stunning range of purple hues, which can range from reddish-pink to red-violet and even display blue-violet shades. Furthermore, these gemstones exhibit an exceptional brilliance, owed to their high refractive index comparable to that of ruby, and boast a remarkable hardness and resistance to chemical reagents.

Rhodolites - like all gemstones of the garnet group - are not subjected to any treatment to artificially improve their properties.   

Rhodolites are very interesting gemstones not only commercially, but also from a gemmological point of view: 

In an extensive study, LIND et al. (1998) examined the absorption spectra of rhodolites and were able to explain the colours of rhodolites from different localities with the help of three different spectral types:  

Rhodolites from Naktamunda, Orissa in India have a unique quality of being very clear and transparent in the blue-violet and red parts of the color spectrum. This gives these rhodolites a special blue-violet purple color that is highly regarded in the gemstone and jewelry industry. They were known as "Naktamunda rhodolites", but have disappeared from the market. The intensely blue-violet stones from Mozambique, which are often sold under the name "Purple Garnet" or "Royal Purple" can be assigned to the same type.

Rhodolites from the Arusha region in Tanzania have different colour characteristics compared to those from Naktamunda. These rhodolites show a lower transparency in the blue-violet part of the colour spectrum. As a result, they appear more purple with a stronger red component. The colour of these rhodolites ranges from a warm "wine red" to a "raspberry color" and is typical for several occurrences in Tanzania.

Rhodolites from Kangala, Tanzania are a third type. The spectra of these rhodolites show absorption bands which can be attributed to manganese and chromium or vanadium, in addition to the iron bands present in the other rhodolite types. This gives them a brilliant purple color that is highly valued in the gemstone and jewelry industry, and they are known as "Kangala rhodolites".

Depending on their actual chemical composition, some of the Rhodolites of the Kangala type may show pink-orange-red colours, while others can change from a purple-red when viewed under incandescent light to a greenish-blue colour when viewed under daylight. These stones can be considered to be in between "Malaya garnets" and color-changing garnets. These stones have found their own market niche within the community of gemstone connoisseurs and enthusiasts.

Investigations of rhodolites from deposits that have since then entered the gemstone market, e.g. from Malawi, have confirmed this classification.

Further reading:

Lind, T., Henn, U., & Milisenda, Claudio, C. (1998). Vergleichende Untersuchungen an Rhodolithen verschiedener Provenienz. Z. Dt. Gemmol. Ges., 47(1), 53–59.

Lind, T., Mueller, S., Milisenda, Claudio, C., & Schmitz, F. (2015). New deposits of Rhodolite from Mozambique and Malawi. Gemmologie, 64(3/4), 50–51.

The Colour Red: Pyropos and Carbunculi

Nowhere else, colour is captured more persistently than in gemstones, which is why gemstones have fascinated mankind from the beginning.

The human colour receptor cells are preferably triggered by blue, green and red: and these colours are just those that have always been preferred for gemstones throughout the history of mankind.

Looking at it from a historical and cultural perspective, garnet is considered one of the most important red gemstones as people have been using it for adorning for thousands of years. We know from studying ancient graves that people wore garnet necklaces even in prehistoric times.

During historic times, the use of garnets for jewellery purposes can be traced for the last three thousand years or so. The determination of garnet origin in historic pieces of jewellery can be used as indicators of trade relations and trade routes in ancient times.

The translation of the hebrew bible into greek, around 300 B.C. in Alexandria, can be considered being among the eldest literature mentioning garnet gemstones: “Anthrax” is among the twelve gemstones symbolizing the twelve tribes of Israel.

The writings of Theophrastos (372-287 v. Chr.) (griech. Peri lithon  = in latin translation “De lapidibus“ = „About Stones”) and Pliny the Elder (23 – 79 n. Chr.) [ Historia naturalis = „Natural History“]  can be considered being the oldest primary scientific sources. In contrast to later writings in medieval times, which mainly concentrated on allegoric and mythological meanings of gemstones, the writings of these two ancient authors are giving details of observations that are rationally understandable.

The greek philosopher Theophrastos is using the term “anthrax” for describing ordinary coal, which burns in a fire, as well as for red gemstones with high hardness, which do not burn in a fire, but glow like burning coal when viewed in direct sunlight. Although Theophrastos is using the same term, both types of “anthrax “ are clearly distinguished by their different properties.

Based on the mineralogical knowledge at that time, it seems to be most likely that “anthrax” was used as a generic term that embraced several red gemstones such as ruby, red spinel and red garnet.

Due to a note of Theophrastos, that „anthrax“ gemstones were used for seals, it is quite safe to guess that Theophrastos meant red garnet, when referring to the gemstone “anthrax”. In hellenistic times (Theophrastos is a contemporary of Alexander the Great) the usage of garnet as sealstones shows a peak in antique jewellery pieces, whereas no ruby sealstones are known from that period.

Carthago and the city now known as Marseille, at the time important Phoenician trade hubs, are explicitly mentioned as trading centres (not: place of origin) for anthrax. Milet in Asia Minor could be both, place of origin and trade hub.

Pliny the Elder translates „anthrax“ into „carbunculus“, and describes the glowing of “carbunculi” when held against the against the sunrays („contra radios solis scintillare“).

Plinius mentions the same trade hubs as Theophrastos, but adds Ethiopia, Alabanda, Egypt and Arabia. Alabanda, near Milet, is mentioned as a cutting centre.

Authors of the medieval period, namely Marbodus (1035-1123 n. Chr.), Arnoldus Saxo (um 1200 n. Chr) and Hildegardis von Bingen (1098-1179 n. Chr.) mainly concentrate on allegoric or mythological interpretations on grounds of the ancestral literature without adding new knowledge about the properties and occurrences of “anthrax” or “carbunculi”.

But, it also can be mentioned, that the birth of the term “garnet” itself lay in the middle ages:

Albertus Magnus (1193 – 1280 n.Chr.) distinguishes between „rubinus“, „granatus“ und „belagius“ as being three different types of „carbunculi“. It seems likely, that the term “granatus” was derived from “granum”=lat. “seed”, making reference to the red colour of the seeds of pomegranate.

The picture above, which shows a garnet crystal in its mother rock, nicely illustrates the etymological development of the terms garnet, pyrope and carbuncle: Fiery-eyed (pyropos) grain (granum), glowing in the rays of the sun like burning coal (carbunculus).

During the renaissance period, mineralogy and metallurgy began to flourish and were recognised by European emperors as an important source of wealth for their growing economies.

Not surprisingly, the exploration of gold and silver ores had top priority, but the rulers also sponsored specifically the search for gemstones and have paid close attention to the development of fine jewellery and gemstone cutting and carving.  

Pyrope and Almandine

Only after routine chemical analysis of minerals in the 19th century had been established, the separation of red garnets into two mineral species was possible:

Pyrope, a magnesium-aluminum-silicate

and

Almandine, an iron-aluminum-silicate. 

With the advancement of scientific knowledge it was also found, that all natural garnets need to be described as mixtures (solid solutions) between ideal end member species, that do not exist in purity in nature. For practical reasons, a solid solution between several end members will be named after the “majority” end member, e.g. Pyrope or Almandine.

Fascinating Garnets

The unbroken popularity of garnets can be traced back to their outstanding gemstone properties: garnets are robust against external environmental influences, have a great hardness and high refractive index, which provides for a perfect brilliance of cut garnet gemstones.

The natural red colour of garnets is pleasant when viewed under natural sunlight as well as in incandescent light in the evening.

Both brilliance and colour of gemstones are brought to its optimum by perfect cutting. Knowledge and experience of the gemstone cutter are of utmost importance when it comes to design the path of light through the gemstone in order to achieve the perfect combination of brilliance and colour perception.

Traditionally, garnets are cut as “rosecut”, which provides for many reflections at the surface of the stones, as well as cabochons.

Very often, garnets are cut in “brilliant cut” with table facet and facetted backside. This cut provides for many internal reflections at the backside of the stone, before exiting through the table facet and reaching the eye of the observer. This cut is preferred for garnets of high transparency.

Modern gemstone cuts quite often use a combination of traditional elements, mixed with completely new ideas, in order to achieve special light effects and therefore are used for individual jewellery creations.

The mystery of garnet colour, fire and lustre has stimulated the creativity of jewellery designers for centuries, again and again.

New garnet occurrences that were discovered in the middle of the 20th century provide high quality rough material for this classic red gemstone.