By Bill Shelton
Color in Beryl
We should consider the fact that a pure beryl has no color-causing element in its formula. Beryllium, aluminum and silicon do not turn out to be among the various elements that usually impart color to a mineral. In nature, we can occasionally find examples where beryl is identified as white, colorless of nearly colorless. The examination of some of these samples shows the absence of a few elements that are typically present in beryl which does exhibit color. We should be especially aware of iron, manganese and chromium because these elements do impart color to the vast majority of specimens we see in collections and dealer stock.
Let’s start out with the variety emerald – the color we usually see is attributed to a trace of chromium in the traditional sense and more recently to vanadium or even a combination or the two. In one analysis, an emerald from Chivor, Columbia was noted to have 0.14% (by weight) chromium. Another example, from Rhodesia, was recorded as having 0.28 to 0.59% chromium which can be compared to a further sample from the Ural mountains in Russia where results state .04 to .38% chromium were present. So, we should conclude there is quite a range of values reported but what the absolute limits may be are not clear and may be subject to change with more reports being issued all the time. Consider the unusual data for a piece from Zambia: it has a value of .01 for vanadium, .07 for chromium and .73 for iron. In spite of the mixture present, the color is said to be a nice emerald green. Analytical results suggest the values for chromium in synthetic emeralds may be higher than the values seen in natural examples.
Red beryl, which has been assigned a few other names, is a rare variety known from a handful of localities. Crystals are small and can be pale to intense in color. The data available indicates the color is due to the presence of trace amounts of manganese. One report lists the value at .08% which can be compared to another variety, morganite where the values are often lower. Chemical testing shows red beryl may contain traces of titanium, zinc, tin, chromium, cesium, lithium, rubidium, boron, zirconium, niobium, lead and others. I dare say beryl from most other places does not have a list like this; it also is odd that red beryl has essentially no water while most all pieces of other types will have water from .76 to over 2.5%. The most noted location seems to be the Wah Wah Mountains, Utah but even the Topaz Mountain area has been noted as a source of similar pieces. The common associates in the area include bixbyite, hematite, calcite, fluorite, garnet, hyalite, pseudobrookite, quartz and topaz.
Morganite, a pink variety of beryl, is noted to contain small amounts of manganese and the color is attributed to it. Besides containing manganese, morganite often also has alkali rich chemistry and this does not seem to affect color but may influence the habit (tabular) that is common in this variety. While an orange colored type exists, it may be a bit unstable and reports say that these pieces will slowly alter to pink with exposure to light. When a yellow tinge is present in morganite, heat treatment may remove it and improve the color to a better pink.
Aquamarine may be the most desired variety for gemstones due to acceptance by customers, availability and price. Here, color is attributed to iron; blue shades are caused by iron 2 in the channel site and this is the classic color for aquamarine. Some sources also include green in the aquamarine sector and then we find color is due to iron also. The difference is that both iron 2 and iron 3 are present. This green color seems to be especially common in both ordinary beryl and gem grade material. Heat treatment may improve the green color to a better blue because the iron 3 is driven off and only blue will remain. You may not be aware of it, but heat treatment is far more common that one may think.
If a beryl contains only iron 3, it will be yellow and perhaps called heliodor. We find this variety to be less common than aquamarine and less desirable for gem use. The color may range from golden to brownish yellow and the rarely seen deep shades are very pretty but still not widely offered as gems. Some greenish samples are also sold as heliodor and here we can quibble about exactly what colors are acceptable under this variety – it seems to me that as one might expect, the definition for any variety may be a bit flexible.
I referred to colorless and white beryl in the beginning – the varietal name goshenite is used here and it implies there is no trace element chemistry causing color in the example. Well, as usual, it may be a bit more involved since iron 2 in the octahedral site and iron 3 in the channel site do not impart color to the mineral. The presence of alkalis and other trace elements also do not add color. So, a clever individual will see that the so-called pure beryl may just be lacking chromophores and not be technically pure. An indirect piece of evidence is hidden in fluorescent data for beryl. Here, mostly we find all types are inert or usually so. Some goshenite, some emerald and a few morganites do fluoresce. These are the very ones that are likely to be relatively iron-free. When we consider the varieties aquamarine, heliodor and red beryl, they are all inert to fluorescent light. Green beryl seems to behave in the same manner. As you may know, iron is often described as quenching fluorescence in a great many mineral species.