leaves towards it, chaff, and the light fringe of tissues". Amber has a long history of use in China, with the first written record from 200 BCE. Early in the 19th century, the first reports of amber found in North America came from discoveries in New Jersey along Crosswicks Creek near Trenton, at Camden, and near Woodbury. Composition and formation Amber is heterogeneous in composition, but consists of several resinous bodies more or less soluble in alcohol, ether and chloroform, associated with an insoluble bituminous substance. Amber is a macromolecule by free radical polymerization of several precursors in the labdane family, e.g. communic acid, cummunol, and biformene. These labdanes are diterpenes (C20H32) and trienes, equipping the organic skeleton with three alkene groups for polymerization. As amber matures over the years, more polymerization takes place as well as isomerization reactions, crosslinking and cyclization. Most amber has a hardness between 2.0 and 2.5 on the Mohs scale, a refractive index of 1.5–1.6, a specific gravity between 1.06 and 1.10, and a melting point of 250–300 °C. Heated above , amber decomposes, yielding an oil of amber, and leaves a black residue which is known as "amber colophony", or "amber pitch"; when dissolved in oil of turpentine or in linseed oil this forms "amber varnish" or "amber lac". Molecular polymerization, resulting from high pressures and temperatures produced by overlying sediment, transforms the resin first into copal. Sustained heat and pressure drives off terpenes and results in the formation of amber. For this to happen, the resin must be resistant to decay. Many trees produce resin, but in the majority of cases this deposit is broken down by physical and biological processes. Exposure to sunlight, rain, microorganisms, and extreme temperatures tends to disintegrate the resin. For the resin to survive long enough to become amber, it must be resistant to such forces or be produced under conditions that exclude them. Fossil resins from Europe fall into two categories, the Baltic ambers and another that resembles the Agathis group. Fossil resins from the Americas and Africa are closely related to the modern genus Hymenaea, while Baltic ambers are thought to be fossil resins from plants of the family Sciadopityaceae that once lived in north Europe. The abnormal development of resin in living trees (succinosis) can result in the formation of amber. Impurities are quite often present, especially when the resin has dropped onto the ground,