What does the color of the Gems depend on and why some change color?

We know that gemstones are the highest expression in nature of some rare minerals.

The color of the gems depends on the light transmitted and absorbed by the crystal of the mineral. When we talk about the crystal we go down to the atomic level, we talk about the arrangement and type of atoms that compose it.

The atoms that cause color can be part of the chemical formula of the crystal or be present as impurities, often in fact the coloration depends on the chromophores or centers of color that are substantially defects in the crystal reticulum. You can have a color center due to the presence of some foreign atoms in the crystal or due to their lack but also due to the lack or excess of electrons in some ions. The lattice defects occur in nature due to the natural radioactivity to which a mineral is subjected during the millions of years and the geological eras spent in its genesis.

An example of chromophore is the chromium ion which can be present both in the corundum lattice becoming responsible for the ruby ​​red color and in the beryl reticulum in which case it will be the cause of the green color of the emerald variety.

Also the particular effect of the color change present in some mineralogical varieties is the result of the presence of ions within the crystalline lattice and the way in which they absorb and reflect light.

The color change is typical of Alexandrite (variety of chrysoberyl) but can also be found in other mineralogical species such as corundums or sapphires, garnets and spinels.
ex: Sapphire Changing

The color of Alexandrite and its peculiarity of color change are due to the presence of chromium ions.

From a spectroscopic analysis it was discovered that Chromium produces in the rubies an absorption band around 550 nanometers and in emeralds around 600nm while in alexandrite it produces an absorption band around 580nm, a middle ground between the ruby ​​and the emerald or balanced between red and green.

Alexandrite changes color depending on the light source to which it is exposed, whether it is a cold or warm light source. Cold or morning light contains large percentages of blue-green while incandescent or hot light contains high percentages of red light.
The chromium present in alexandrite, exposed to cold or diurnal light, is responsible for the green color, while in the light of a red incandescent lamp. Even our eye does its part, exposed to a source of cold light the crystal reflects both green and red but the percentage of green is greater to the point that our eye only perceives the green while at the incandescent light only the red color.

Also the effects of fluorescence (eg in diamonds) and phosphorescence of some minerals may depend on the ions they contain, their electrons can change the position inside the atom and their state depends on the absorption of energy. The transition of these electrons can be temporary and triggered by the absorption of light radiation, that is from the different sources of lighting used.