Amber

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Amber
Chemical composition C-H-O compound (fossil resin)
Crystal system Amorphous
Fracture Conchoidal, brittle
Hardness 2-2.5
Refractive index 1.54
Specific gravity 1.05-1.09
Lustre Resinous
Mexican amber
Photo courtesy of Dave Gibson

Amber image gallery

Amber is the fossil resin from a pine tree that flourished in the Baltic region (and other localities) some 25 to 60 million years ago. Globs of this aromatic sap poured down from prehistoric trees, often trapping insects, twigs, bark and leaves. Amber is one of the few gemstones of organic origin. In ancient India and Egypt, amber was burned as an incense, believed to purify the surrounding area. Amber is abundant along the shores of the Baltic Sea where it is mined extensively from Tertiary glauconite sands that are from 40 million to 60 million years old.


Diagnostics

Amber is often very hard to seperate from its imitations due to overlapping physical and optical properties. When no internal features are diagnostic, one can, usually, only fall back on destructive methods like the hot needle test and ether/aceton.

Color

Blue amber in reflected and transmitted light


Amber occurs in the colors white, yellow, orange, red, brown, blue, black, green and has a typical resinous luster.

Blue and green amber is found only in the Dominican Republic. This blue (or green) color is only seen in reflected light, in transmitted light it will show a more common amber color (such as yellow, orange, red). Under UV light these blue gems give a strong cobalt blue fluorescence.
The cause of color of these blue and green gems is believed to be from fluorescence of UV and violet light from natural sunlight.

Delocalized electrons on organic molecules are the cause of the other colors.

Diaphaneity

Transparent to opaque.

Refractometer

Amber has a refractive index of ~1.54 and is singly refractive.
Copal and some plastics are in the same range of amber gemstones.
Glass and chalcedony also have a similar refractive index to amber.

Specific gravity

The specific gravity varies from 1.05 to 1.09. Amber with many gassbubbles are in the lower range.
Amber floats in a saturated saline solution. This is the reason why amber is found on many shores around the Baltic Sea, it floats on sea water.
As with the refractive index, copal and some plastics fall in the same range.

Hot needle test

When a needle is heated and then brought to a piece of amber, it will give a pine tree odor, while the plastic similants will smell like burned plastic.

Ether

When a small drop of ether or acetone is placed on plastic or copal, it will leave a sticky surface while they have no effect on real amber.

Polariscope

Amber is singly refractive and will show anomalous extinction (ADR) and stress colors between crossed polarizers, as will some plastics.

Magnification

  • Insects (usually extinct species) and other flora and fauna that got trapped by the resin.
(These can be mimiced in plastic)

Electricity

The greek word for amber is "electron" and "electricity" is named after this gemstone.

When rubbed with silk, amber will get a negative electric charge while glass will be positively charged.
Plastics will also be charged negatively like amber.

Treatments

Heat treatment

Alot of amber contains many gas bubbles that give it a hazy appearence. Slow heating between 150-180° C, followed by slow cooling, can diffuse these bubbles out.
When the cooling takes place too rapidly, typical leaf like stress inclusions form (these are know as "sun spangles") and these inclusions are, more than often intentionally, mistaken for prehistoric leaf inclusions.

Another type of heating involves laying the specimens in a sand filled iron pot and heat it over an extensive period to darken the amber. This gives it an "antiqued" look. Natural amber may darken naturally over time due to air oxidation.

Other treatments

  • Dying
  • Coating
  • Foiling

Imitations

Reconstructed amber

One type of amber that one may encounter is reconstructed amber. This type of amber is also marketed as "ambroid".
Clear fragments of amber are carefully selected and heated upto ± 180° C under high pressure of +350 bar in a reducing atmosphere with sometime linseed oil added. Under these conditions the fragments fuse together and some organic dyes may be added to the process to influence the color of the resulting solid block.
These gemstones may whiten over time opposed to natural occuring amber which will darken.

The following inclusions may be seen:

  • flow lines
  • elongated bubbles

A strong bright blue fluorescence might be observed.

Copal

Copal is also a natural occuring fossilized resin which can also be artificially created. The resin has been burried in the earth for a conciderably shorter period and its hardness is lower than that of amber.
The best way of descriminating between copal and amber is by placing a small drop of ether or aceton on the gemstone. This will create a sticky surface on copal, while it has no effect on amber. This is a destructive method though.
The fluorescence of copal is much whiter than that of amber.

Plastic

Plastic is often used to imitate amber. Even complete insects and/or small animals like salamanders are embedded into the mould to mimic natural fauna inclusions.
When the hot needle test is applied to them, it will smell acrid instead of piney.

If you are offered a piece of amber with a complete salamander at low cost, there must be something wrong as they would fetch several ten-thousands dollars when genuine.

Occurrence

  • The areas around the Baltic Sea (Poland, Denmark, Finland, Sweden, Germany, Latvia, Lithuania)
  • The Dominican Republic
  • Mexico

Sources

  • Gemstone Enhancement (1984) - Kurt Nassau ISBN 0408014474
  • Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
  • Gem Reference Guide (1995) - GIA ISBN 0873110196

External links

Museums