hard, translucent, brownish-yellow substance that is the fossilized resin of ancient trees. It often contains fossil insects.
Did You Know? The plot of the movie Jurassic Park turns on the extraordinary ability of amber to preserve ancient life as miniature fossils.  In the movie, scientists extract dinosaur DNA from blood in the stomach of a mosquito that was trapped in amber during the Mesozoic Era. The scientists then use the DNA to create clones of the dinosaurs that end up terrorizing the park and the movie audience.  What is amber, and how does it preserve such delicate tissues for millions of years? Certain trees, especially conifers, produce a sticky substance called resin to protect themselves against insects. Normally, it decays in oxygen through the action of bacteria. However, if the resin happens to fall into wet mud or sand containing little oxygen, it can harden and eventually fossilize, becoming the yellowish, translucent substance known as amber. If any insects or other organisms are trapped in the resin before it hardens, they can be preserved, often in amazing detail. While amber may sometimes preserve fragments of the DNA of the enclosed organisms, fossil mosquitos would not contain enough dinosaur DNA to actually create clones, and the sight of dinosaurs again roaming the Earth, even in an island park, remains part of science fiction rather than real science.

A gemstone or gem (also called a fine gem, jewel, or a precious or semi-precious stone) is a piece of mineral crystal, which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks (such as lapis lazuli) or organic materials that are not minerals (such as amber or jet), are also used for jewelry, and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their lustre or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone. Wikipedia

The diamond, ruby, sapphire and emerald are precious stones. The RockTumbler website has a chart for semi-precious stones.