There are currently 110 elements recognised by the International Union of Pure Applied Chemistry (IUPAC). These elements are displayed in the form of a matrix called the Periodic Table. The current form of the Periodic Table, including the new group numbers (1-18), was agreed to by the IUPAC in 1985.
The periodic table is the most important chemistry reference available. It arranges all the known elements in an informative array. Elements are arranged left to right and top to bottom in order of increasing atomic number. Order generally coincides with increasing atomic mass.
All minerals, therefore gems, are made up of elements. Some minerals have a simple chemistry with just one or two elements, such as Diamond which consists only of Carbon [C] or Quartz consisting of Silicon Dioxide [SiO2]. Others, such as Tourmaline [NaLi2.5Al6.5(BO3)3O18(OH)4] or Axinite [(Ca, Mn, Fe, Mg)3Al2BSi4O15(OH)], are very complex and may vary in chemical composition from one crystal to another. The properties of a gem are determined by and affected by its chemical composition. These properties are things like crystalline structure, crystal shape, optical properties, cleavage, hardness and color.
While some of these properties are not immediately evident when looking at a gem, color is very obvious. The beautiful rose-pink to orange-red colors of a Rhodochrosite gem are due to its manganese (Mn) content. A Demantoid Garnet is colored deep green by its chromium (Cr) content. Gems from the Beryl family all have the same basic chemistry but are different colors due to the various trace elements added. Aquamarine gets its blue-green color from traces of ferrous iron (Fe). Bixbite (red Beryl) is colored red by manganese (Mn). Emerald is colored green by the addition of chromium (Cr). Heliodor is yellow from ferric iron (Fe). The pink variety of Beryl, Morganite, gets its color from slight amounts of manganese (Mn). Goshenite however, is colorless because it is pure Beryl and not colored by any impurities.