Amber can encase your wooden floor, but it can't encase a giraffe

Among gemstones, amber and pearl count as a rather unique category - they belong to"organic gemstones", meaning that their main component is organic matter.

In the history of chemistry, the original definition of "organic matter" was exactly its literal meaning - substances and their derivatives that can only be synthesized by organisms (living things). For example,pearlis the secretion of some bivalve mollusks; whileamberis the fossil of resin secreted by certain trees.

Amber encasing various small insects. Image: Didier Descouens / wikipedia

According to current research conclusions, there are many kinds of trees that can form amber, among which possiblymost are gymnosperms(such as pines, cypresses, and firs), yet some angiosperms can also form amber. Among them, the famousDominican amber(as well as similar Mexican amber and other American ambers) has already been confirmed to be formed from the fossilized resin of an extinct leguminous plant, Hymenaea protera (perhaps it could be given the Chinese name "Dominican amber bean"). Although the Dominican amber bean is extinct, scientists have found that it is a close relative of another living plant -amber bean(Hymenaea verrucosa).

Who is the best at secreting resin

This extant amber bean belongs to the genus Hymenaea, and it is so named because each of its leaves is composed of a pair of leaflets,like twin sisters. Among the 14 extant tree species in the genus Hymenaea, 13 are still found in tropical America, and only the amber bean most closely related to the Dominican amber bean is found in East Africa. Because of this geographical difference, some scholars once classified it as a separate genus -the genus Amber Bean（Trachylobium）。

Note that each leaf is composed of a pair of leaflets. Image: Dinesh Valke / wikipedia

This America-East Africa disjunct geographic distribution of the genus Hymenaea is not difficult to explain. When this genus originated, the Americas and Africa had at mostjust split from a larger Gondwana, so plants of this genus existed on both continents. As the Atlantic continued to expand, the Americas and Africa became increasingly distant from each other, and the genus Hymenaea developed independently on both sides. The American "lineage" flourished somewhat more, while the African "lineage" fared more poorly, ultimately forming today's transoceanic geographic distribution pattern.

Finding evidence in fossils

How did scientists confirm that the amber bean and the Dominican amber bean are close relatives? First, on the preserved leaf fossils of the Dominican amber bean, people can see clear leaf veins similar to those of the living amber bean. Although in general plant identification leaves are not as quick to reveal a plant's identity as flowers and fruits, for paleobotanists leaves are a very important organ,and the morphology of leaf veins contains rich information。

Leaf fossils of the Dominican amber bean; the fourth image shows magnified leaf veins. Image: George O. Poinaret al./Experientia (2005)

Second, although for an ancient plant from thirty or forty million years ago most of the DNA in its cells has already degraded, paleobotanists still managed to extract some chloroplast DNA fragments from it. After sequencing and comparing them with extant tree species, they finallyfrom both morphological and molecular aspectsdetermined the kinship between the two.

After tens of millions of years of evolution, tree species of the genus Hymenaea are stillknown for secreting resin. Hymenaea resin is customarily classified as a type of resin called copal. Besides the genus Hymenaea, theresinhorseshoe fruit(Protium copal) of the Burseraceae family is also a source tree for this type of resin. For the word copal, there have historically been various translations, such as keba resin, kebo resin, gupai resin, and so on. Of course, the most magical is guba resin (note: pronounced guba), for which two characters that ordinary input methods cannot type (and that cannot be displayed on mobile phones either) were actually specially coined.

The trunk of an amber bean tree is exuding resin. VE McCoyet al./ScienceDirect (2017)

Besides amber, it can also make varnish

The confusion in the translation of copal shows how unfamiliar Chinese people are with it. According to the records in Flora Reipublicae Popularis Sinicae, two species of the genus Hymenaea are cultivated as economic crops in South China and Taiwan, China, namely Hymenaea courbaril and the amber bean (called "warty-fruited Hymenaea" in Flora Reipublicae Popularis Sinicae). However, cultivation is one thing; in the end it was not promoted as a widely planted economic tree species.

So what use is this copal? It turns out that it isan important component of varnish. Varnish is made by dissolving solutes such as resin in a liquid, volatile oily solvent (such as turpentine). When varnish is evenly applied to the surface of an object, the oily solvent gradually evaporates, and the resin and other solutes gradually dry and "solidify" into a hard, waterproof, and glossy layer, which both protects the surface of the object below and makes it look beautiful.

A kitchen wooden floor coated with three layers of varnish, waterproof and glossy. Image: avlxyz / flickr

In the 19th century, Westerners discovered that varnish prepared with amber bean resin had excellent performance and was especially suitable for application to wood, so they imported it in large quantities from Africa, especially from Zanzibar Island in Tanzania, the largest production area of amber bean. It was not until the 20th century, as artificially synthesized chemical paints began to be applied, that demand for copal varnish continuously declined. In China,tung oil and lacquerhave been two widely used varnish-like coatings since ancient times, and before copal varnish had time to replace them, China had also entered the era of chemical paints. This is why it is no wonder that we are unfamiliar with amber bean and copal varnish today.

However, in the gemstone world, copal is still very famous, because somecopal "sub-fossils"buried for only a short time (for example, only a few million years) are often sold as fully fossilized amber, so that how to distinguish true amber from fake copal amber has become a lesson that everyone who wants to handle or invest in amber, this kind of organic fossil, must master - just as people who like organic food must also pay attention to whether what they are eating is real organic food or fake organic food.

This article is the 234th article of the 4th year of Species Calendar, from Species Calendar author @Liu Su.

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