I hope most of my readers realise that amber is not really "made" as such. It has been around for hundreds of millions of years and is a product of a long process.
Amber is fossilized tree resin (not sap). Hundreds of millions of years ago huge coniferous forests covered big parts of what is today Baltic sea and its environs. Resin from these trees (in our region its Pinus succinifera and similar trees) seeped down sometimes trapping insects or other organic matter (this way forming so called inclusions) and then solidified. Geological changes caused parts of the forest to be buried under the ground or later even water and the process began.

Small raw amber nuggets

Molecular polymerization, resulting from high pressures and temperatures produced by overlying sediment, transformed the resin first into copal. Sustained heat and pressure drove off terpenes and resulted in the formation of amber.
There are several conditions which had to be present in order for resin to become amber.

The starting resin had to 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, and temperature extremes tends to disintegrate resin, and the process is assisted by micro-organisms such as bacteria and fungi. For resin to survive long enough to become amber, it had to be resistant to such forces or be produced under conditions that exclude them.
Other kinds of amber came to be in a similar way, only the resin came from different trees, thus resulting in a different chemical composition.

This is the reason why only Baltic amber is considered to have medicinal properties due to one of its components -- succinic acid. Teething necklaces and bracelets (only made from untreated Baltic amber) are used for this reason as well. If you are choosing a teething necklace for your child -- be careful, a lot of amber that is nicely transparent (especially bigger sized beads, uniform in shape) is usually heat treated amber -- it is boiled in hot oil to become transparent. But unfortunately during this procedure it loses most of its succinic acid, which is the healing bit of amber.

Personalized amber teething necklace by Drop of Amber