Amber versus Copal
By Scott Anderson
I think that the real tough part on defining amber is that the definition varies, depending on who is defining it (and also who is listening!)
It appears to me, that there is no clear cut, all encompassing definition of amber that would satisfy the several areas of science interested in this definition. Basically, people will use the chemical definition and ignore the physical definition and vice versa. This does not even count the inclusions yet.
From a Geology/Gemologist perspective: Amber is a given name for resin that 
has been "amberized" through sufficiently cross-linking of the long carbon chain organics of the resin (polymerization, forming stronger bonds) and expulsion of volatiles. There is no clear age given or even defined. It has been suggested that the process take at least a few million years, but with any chemical process, other factors are key to the rate. The typical definition of amber is based more on the resulting end physical properties that the specimen exhibits due to the amberization process. It's what many of us probably use, we just don't realize it. Copal will craze and melt fairly easily, while amber is harder, slower to craze, and will not melt. As many of you know, crazing is caused by the release of volatiles within the resin (also surface interactions with oxygen). As cross-linking increases over time, volatiles are also driven off. Internally, amber is more stable than copal for two reasons - less volatiles and more cross-linking.
From a geology point of view, all copal, amber, and resins are generally classified as resinites and/or fossil resins. There is no desire to classify them in name any more. There are distinctions in resinites, being broken into families based on the chemical polymerization process that occurs and the bonds/bond structures. This is outlined in the book Amber, Resinites, and Fossil Resins by Ken B. Anderson & John C. Crelling. Gary Platt's website does a good job covering the chemical definition and transformation of amber.
Undoubtedly, many of you have probably seen the article by Dr. Robert E. Woodruff, Emeritus Entomologist, Florida State Collection of Arthropods, that calls all resins copal, based on its Aztec source. Here's a link: http://www.multistoneintl.com/mineralspecimens/amber_woodruff.htm . He focuses on resins and the inability to date them. He's right on many aspects. From a chemical perspective, copal and amber are composed of resins. Chemically, they are very similar from start as fresh resin to end as amber. When cross-linking (polymerization) occurs, the chemical structure is not altered. The long carbon chains just become intertwined, forming a more stable forms through bonding. They are the same chemical formula, just weakly bonded together forming very long chains. Keep in mind, resins are unique to their source material. This allows for examination of resins to determine sources. It still does not define amber though. Which, is really his main point.
Short story conclusion - If one considers amber and copals from chemical and geologic perspectives, all terms are nearly
interchangeable.
It's the physical characteristics that really define the distinction between amber and copal. We always cite amber properties as our defense, while people who call copal amber cite the chemical properties and/or age uncertainties. Unfortunately, again, there is no clear cut definition that satisfies everyone.
What we really need, is a clear definition of amber that sets forth standards from a chemical, physical, and biological basis. I don't feel that age is enough to distinguish amber from copal. Consider this... If amber is amber because of the extent of cross-linking and expulsion of volatiles, there should be a way to measure this. Fresh resin is composed of organics and volatiles, in no organized structure. It's a sticky liquid. Copal and amber are amorphous solids (meaning that it lacks a crystal structure). If long chained bonds are formed (well, bonded together) and intertwined, something like x-ray crystallography may reveal its rough "crystal" structure. Maybe we'll see a transition of bond separations (from none in resin to whatever in amber) from long chained molecule to molecule. This can be used on top of already observable physical difference, and presence of recent vs. extinct taxa.
Scott Anderson graduated from Penn State in 1995 with Honors and High Distinction with a BS in Geosciences. He is a Senior Geologist at Tetra Tech NUS in Pittsburgh, where he primarily performs computer modeling (contaminant fate and transport). He began collecting amber when he was 14. He now has 19 different orders of insects and many non-insect orders. He specializes in Dominican amber inclusions.
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