Blue amber Most amber is the yellow-orange colour of tree…

Blue amber Most amber is the yellow-orange colour of tree…

Blue amber

Most amber is the yellow-orange colour of tree resin, but a rarely found variant exhibits this lovely blue colour. When light is shone through it or reflected off a white surface behind it, it transmits the normal resiny colour. When viewed in sunlight or against a dark background, this wondrous blue glow appears. It is caused by fluorescence, with the UV component of the ambient light being absorbed and rereleased as electrons jump to a higher energy state and then give off the absorbed energy as blue light.

A variety of trees throughout life’s history have produced resins that subsequently fossilised, but, so far, only those that came from a species called Hymenaea protera display this effect. Its resin contains a variety of hydrocarbons that fluoresce in weak intensity UV light. Crude oil and normal amber fluoresce too, but the latter needs higher excitation energies than daylight provides in order to glow. When placed in a UV darkbox, blue (and the related green) amber glows much more intensely than other shades. The source tree is known from the large amount of flowers preserved within the resin, that are similar to those of a still living descendant of these trees.

The ageing of tree sap into amber is a complex chemical process, involving loss of volatile components and many polymerisation reactions (forming of long chains) between organic molecules in the original sap. In this case, highly fluorescent polycyclic aromatic hydrocarbons were produced, causing the blue glow. Their precursors are thought to be molecules of burnt tree called anthracenes, the consequence of long burnt out forest fires. Red and yellow amber from the same sources do not contain these chemicals, and so display this gem’s more traditional shades.

Most of it comes from the Dominican Republic, though rare specimens of Mexican (Chiapas) or Indonesian amber also exhibit the phenomenon. Dominican amber in all its hues is aged 25-40 million years old, and is both much more prolific (about 10x) in fossil inclusions such as insects and plant matter and more transparent than the better known Baltic material. It was baked into amber after being buried by sediments in a deep lagoon before being uplifted to the surface during the Caribbean plate’s ongoing collision with America. The mines are now found in the hills of the Cordilleras Septentrional and Oriental .

Imitations have been made by dyeing, but they do not exhibit the eerie milky glow that characterises fluorescence, and the dye should be visible concentrated in fractures in a handlens. Checking for dye should always be done when purchasing any coloured amber, and the dyed material has become a jewellery phenomenon in its own right. Other common imitations include plastic, which does not have the milky quality (also shared by the more highly fluorescent diamonds), and should sink in a glass of salt saturated water, while amber should float (as indeed it does between the Baltic and the East coast of England). Dominican amber is less commonly treated than some others, that are routinely boiled in oil to hide cracks and air bubbles (round disc like fractures within the stone are one easily seen consequence of this treatment.

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Loz

Image credit: Vassil, Wikimedia Commons

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