By Yaĝé Enigmus
Due to the high degree of similarity between the chemical structures of various garnet minerals, there exist garnet gemstones which owe their beauty to a mixing of different garnet species within individual crystals. These mixed garnets are sometimes referred to as “hybrid garnets”, and while they may be classified as varieties of the major gem garnet species, they are typically discussed in distinct terms from their parent minerals when used as gemstones.
The most common and most popular hybrid garnet gemstone is a blend between pyrope garnet and almandine garnet which displays colours ranging from wine red to rose pink. This garnet gemstone is known as “rhodolite garnet”, a name derived from the Greek word “rhodon” meaning “rose-like” in reference to its often rosy appearance. Mineralogically regarded as a variety of pyrope garnet, rhodolite garnet is primarily pyrope garnet in composition with a significant percentage of almandine garnet present, typically occurring as crystals with a magnesium-iron ratio that approaches 70Mg/30Fe. The thinly dispersed ferrous iron (Fe2+) constituent of rhodolite garnet is responsible for the light reds and pinks exhibited by this gemstone, with the occasional trivalent chromium (Cr3+) impurity also contributing to the rich appearance of some gems.
First identified by Western scientists in the Umba River Valley of Tanzania, the pyralspite garnet hybrid now known as “malaya garnet” (or "malaia garnet") has become its own distinct garnet gemstone. These stones were originally mined alongside rhodolite garnet crystals and were mistaken for rhodolite garnet of unideal colour, often being cast aside during the grading process; such stones eventually were recognized as their own distinct gems and given the name “malaya” which is derived from a Swahili word meaning “misfit” or “outcast”. Although these stones were initially thought to be primarily pyrope in composition, further investigation revealed that these garnets had a unique structure that contained notable quantities of spessartine garnet and almandine garnet, with a small number of stones also containing limited quantities of grossular garnet and andradite garnet. Due to the highly variable composition of malaya garnet, gemstones may exhibit an equally variable number of colours, including rose pink, peach pink, peach orange, brown, beige, and soft golden yellow.
Faceted pyrope-rich malaya garnet gemstones from Tanzania. Image: Gemstone Magnetism
There exist hybrid garnet gemstones which appear to exhibit different colours under different lighting conditions. These stones are known as “colour-change garnet”, and most are pyralspite garnet hybrids. The majority of pyralspite colour-change garnets are primarily pyrope-spessartine in composition with small amounts of almandine garnet present in their structure, however it is the impurities these garnets contain that give them the colour-change phenomenon for which they are named. Typically small impurities of trivalent chromium (Cr3+) and/or trivalent vanadium (V3+) are the key chromophores responsible for the increase in red hues seen when these garnets are viewed under traditional artificial lighting conditions. The colour of these garnets varies considerably depending on the ratios of each garnet species constituent, but yellow-to-red, purple-to-green, pink-to-orange, and brown-to-red shifts are all well-known occurrences, with colour shifts from red-to-green possessing the most value. In extremely rare cases, colour-change garnets may display blue hues under the right lighting conditions, with such stones being the only garnet gems known to possess the capability of exhibiting a blue appearance.
A faceted colour-change garnet gemstone from Madagascar displaying multiple hues under different lighting conditions (L - Daylight; M - Incandescent; R - LED). Image: Gemstone Magnestism
Grandite Garnet (and Mali Garnet)
A hybrid garnet of the ugrandite series, the gemstone known as “grandite garnet” is primarily grossular garnet in composition with varying amounts of andradite garnet present. Mineralogically regarded as a variety of grossular garnet, this garnet gemstone is named after the two garnet species of which it is composed. Typically occurring with an iron-aluminum ratio of 8Fe/92Al or greater, grandite garnets range in colour from deep green to burnt brown, but most often occur in bright yellow or golden yellow tones, with the presence of andradite garnet in the larger grossular matrix enhancing this gemstone’s lustre and dispersion beyond what is normally seen in grossular garnet. Brown and yellow hues in grandite garnet are caused the ferric iron (Fe3+) in andradite’s structure and in some cases by small impurities of ferrous iron (Fe2+); the green hues seen in some grandite garnets are caused by impurities of trivalent chromium (Cr3+). Grandite garnets have been found all over the world, but the first significant deposit of gem-grade material to be documented was discovered in the country of Mali in 1994; earning the name “Mali garnet”, one that is now sometimes used for grandites from other locations but is typically reserved for stones mined in Mali. The grandite garnets found in Mali were primarily of golden colouration and formed with an iron-aluminum ratio nearing 18Fe/82AL, giving them superior optical properties relative to other grandites.
With such a vast world of garnets, almost any individual is likely to find a gemstone that matches their taste. Despite the prevalence of red garnets, there is no shortage of other colours displayed by this mineral group, and those born in January need not worry about being restricted to stones of crimson hues. Perhaps the most misunderstood contemporary birthstone, garnet gems are truly a gemological adventure.
The Skyjems collection features rhodolite garnets in deep, luxurious hues: