Amber:  the Organic Gemstone

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  • Amber: the Organic GemstoneJOSEPH B. LAMBERT*, ANDGEORGE O. POINAR, JR.Department of Chemistry, Northwestern University,2145 Sheridan Road, Evanston, Illinois 60208, andDepartment of Entomology, Oregon State University,Corvallis, Oregon 97331

    Received May 16, 2001

    ABSTRACTResins are produced by woody plants on a worldwide basis. Wehave found several distinct classes of modern diterpenoid resinsbased phenomenologically on the solid-state 13C NMR spectra ofthe bulk material. Resin fossilizes over millions of years into arobust material sometimes called amber. We have characterizedseveral hundred samples of fossil resin by solid-state 13C NMRspectroscopy. We can relate one globe-spanning group of fossilresins to the modern genus Agathis, based on spectral evolutionover time. A second large group has not been related with certaintyto specific modern plants. Fossil resins from Europe fall into twocategories, the famous Baltic ambers and another that resemblesthe Agathis group. Fossil resins from the Americas and Africa areclosely related to the modern genus Hymenaea. Based on spectraldistinctions, fossil resin found in an archaeological context some-times can be assigned to a specific geographical origin on the basisof its 13C NMR spectrum.

    IntroductionAlmost unique in the mineralogical world is the organicsubstance amber.1,2 Other gemstones are inorganic, butamber is a fossilized form of terpenoid plant resins.Numerous genera of plants all over the globe spontane-ously or as the result of trauma produce sticky substancesthat have been termed resins. These substances have beenharvested and used throughout history as adhesives,coatings, and binding media for pigments. Some suchmaterials give off pleasant or exotic odors on burning andhave been used as incense.

    Through evaporation of volatile components and po-lymerization of dienic functions, resins over geologicaltime can fossilize into extremely hard materials. Becauseof their attractive appearance, their ability to be carvedand polished, and their robust, stone-like texture, suchfossilized resins have been treated as gemstones for

    millennia throughout the world (Figure 1). In the Euro-pean context, these materials have been called amber andhave been associated with the eponymous yellow-browncolor.

    Unworked fossil resin has been found in associationwith Paleolithic dwelling sites in the Old World1,2 (priorto about 12 000 B.C.3). Worked fossil resin has been foundin Mesolithic sites (about 8000-12 000 B.C.). By theNeolithic period (the New Stone Age, associated withfarming, from about 8000 B.C.), fossil resin had clearlybecome a widely traded material. In addition to itsattractive appearance, amber is warm to the touch andattracts other objects on rubbing. It is one of the mostcommonly mentioned gemstones in the writings of Homer(Penelope wore a necklace that was golden, set withamber, like the radiant sun). The Greek word for amberwas elektron. Because of the electrical properties of amber,this word became the root for all English words associatedwith the terms electron and electricity.

    The vast majority of European amber originated aroundthe Baltic Sea, and so trade networks developed to movethe supply to Mediterranean markets. Nonetheless, thereare natural fossil resin deposits all over Europe andadjacent Southwest Asia, so the actual source of a specificartifact cannot be attributed automatically to sources closeto its find site. Schliemann4 contemplated the rich amberfinds at Mycenae by commenting that it will, of course,forever remain a secret to us whether this amber is derivedfrom the coast of the Baltic or from Italy.

    Modern scientific analysis of amber and other fossilresins has proved Schliemann wrong, as spectroscopic

    Northwestern University. Oregon State University.

    Joseph B. Lambert is Clare Hamilton Hall Professor of Chemistry at NorthwesternUniversity. Although his primary areas of research have been in organic chemistry,he has been involved for 30 years in the applications of chemistry to archaeology.He is the author of the book Traces of the Past, which presents the subject fora general audience. He has won the American Chemical Society's Kipping Awardin silicon chemistry, the Society of American Archaeology's Fryxell Award inarchaeological sciences, and the Chemical Manufacturers Association's CatalystAward in the teaching of chemistry.

    George O. Poinar, Jr., spent some 30 years in the Department of Entomology atthe University of California, Berkeley, interspersed with leaves to conduct researchin The Netherlands, Paris, St. Petersburg, New Zealand, and Australia. Hisresearch interests include entomology, nematology, botany, and parasitology.He has authored or coauthored eight books, the last four of which cover variousaspects of amber, especially the reconstruction of ancient ecosystems basedon amber fossils. He currently is at Oregon State University, conducting researchon living and extinct plants and invertebrates.

    FIGURE 1. A box constructed of amber pieces and attached to achain of amber beads, several hundred years old, from the Museumof the Earth, Warsaw, Poland. Photograph by G. O. Poinar, Jr.

    Acc. Chem. Res. 2002, 35, 628-636

    628 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 35, NO. 8, 2002 10.1021/ar0001970 CCC: $22.00 2002 American Chemical SocietyPublished on Web 02/16/2002

  • characterization of the material can indicate geographicalsource. C. W. Beck5 pioneered the use of infrared spec-troscopy for the characterization of amber. In particular,he found diagnostic vibration patterns for materials fromBaltic sources. Mills, White, and Gough6 used massspectrometry to identify numerous components in theether-soluble fraction of Baltic amber. Anderson, Botto,and their co-workers7 expanded the technique by usingpyrolysis gas chromatographic mass spectrometry (GC/MS). Based on the identity of the pyrolysis products, theydefined several classifications of fossil resins (which theycall resinites). Shedrinski, Grimaldi, and co-workers8 alsoused pyrolysis GC/MS, with a focus on identifying amberforgeries. We have employed solid-state nuclear magneticresonance (NMR) spectroscopy with cross polarizationand magic angle spinning (CP/MAS) to characterize bothmodern and fossil resins on a worldwide basis.9 Whereasthe MS approach focuses on identifying botanical differ-ences based on small-molecule components, the NMRapproach uses bulk analysis to examine both botanicaland geographical differences. This Account describes theapplication of NMR to the study of solid resins, withrelevance to both archaeology and botany. We begin withcharacterization of modern resins by NMR, then of fossilresins of known geological provenance, and finally ofarchaeological materials.

    Modern ResinsResins usually are solid when the constituent moleculescontain at least four isoprene units, i.e., when they arediterpenes (C20) or higher. The conifers (order Coniferales)and angiosperms (family Leguminosae) are the mostimportant groups that produce diterpenoid resins.10 Co-nifer families that produce such resins include the Pi-naceae (pines), Cupressaceae (cypresses and junipers),and Araucariaceae (kauri and others). Diterpenoid resinsare particularly prone to polymerization and hence arethe predominant source of fossil resins used for culturalpurposes. Triterpenoid (C30) resins originate primarilyfrom broad-leaved trees and generally are nonpolymer-izing. They have found wide uses as varnish resins forpaintings and as incense. Triterpenoid resins includedammars (subfamily Dipterocarpoideae of the familyDipterocarpaceae), mastics (genus Pistacia of familyAnacardiaceae), and elemis (family Burseraceae, whichalso produces the famous gum resins myrrh and frank-incense).

    It is not often possible to distinguish one resin sourcefrom another by appearance alone. GC/MS characteriza-tion has been very successful at identifying small mol-ecules in either a soluble fraction or a pyrolysate. Ourmodus operandi has been to examine the solid-state 13CNMR spectrum of bulk resins, with two modes of decoup-ling. Under normal decoupling conditions, signals shouldbe obtained from all carbons present in the sample. Inexperiments with interrupted decoupling, signals areselected for quaternary carbons and for some carbons thatare moving rapidly in the solid; other signals are edited

    out. The two spectral modes serve primarily as fingerprintsof the bulk material.

    In this way we have examined modern resins fromseveral continents, including the conifer Pinaceae genusPinus (pine), the conifer Araucariaceae genera Agathis,Araucaria, and Wollemia, the angiosperm genus Hyme-naea, and the triterpenoid family Burseraceae.11 Sampleswere harvested from living trees, except for the Burser-aceae, which was a commercial sample, probably fromthe genus Bursera from Mexico.

    Figure 2 contains five representative pairs of spectra.11

    The spectrum with interrupted decoupling is always theuppermost of the pair. The top two are from Pinusmonticola from California. The most prolific of the Pi-naceae genera, pines probably are the most importantsource of resins in temperate regions of the northernhemisphere. We found nearly identical spectra for all pinesamples, as well as for U.S. Rosin and commercial violinrosin. The most distinctive spectral characteristics are thefive peaks in the saturated region of the (upper) spectrumwith interrupted decoupling.

    The next two spectra from the top are of Hymenaeacourbaril from the Dominican Republic.11 This angiospermgenus is the most common source of resins in tropicalAmerica and East Africa. Its characteristic resonancesinclude the four peaks in the aliphatic region of thespectrum with interrupted decoupling and the strongexomethylene (CdCH2) resonances at 108 and 148(absent for Pinus) of the normal spectrum. Very similarspectra were obtained