When tree resin fossilizes over millions of years, it produces a gemstone known as amber. Cognac amber resembles the rich brown color of French brandy, and darker hues rank among the more sought-after shades of amber, reports ShopGemstones.com.

Unlike mineral rocks, cognac amber feels slightly warm to the touch, and gives off a slight fragrance when warmed. It may contain "inclusions" such as plant debris, pollen and even insects trapped in the tree resin and preserved when it fossilized, which increase its value, according to GemSociety.org.

The Gem Society rates cognac amber--a natural polymer, or plastic--as a fairly delicate stone. Heat, as well as chemicals, alcohol and solvents can damage amber, and its softer texture makes it more susceptible to scratches.

Some people mistake a different form of preserved tree resin, called copal, for cognac amber. But copal's just a few hundred thousand years old, the Gem Society points out, and has less value.