There are different ways in which gems show multiple colors. Some common ones are color change, bicolor, and dichroism.

  Color Change gems show different colors based on the type of lighting they’re under. These are denoted with a slash.  A great example of color change gems is lab Alexandrite, which is listed as “teal/red”. This is because one color is visible in sun light, whereas another color is visible in florescent light. More on viewing color change later!

Bicolor gems clearly show two distinct colors in all types of lighting, also listed with a slash. For instance, Ametrine shows purple and yellow with a clear color break, so the color is listed as “purple/yellow”.

  Dichroism is present within gems that show different colors based on the angle at which you view them and are also listed with a slash. Reflection of this effect causes faceted gems to sparkle with both colors. For example, Andalustite, which are reddish in one direction and greenish in another.

  Blended color gems appear as a mixture of the two colors and are listed with a dash. For example, Citrines are described as yellow-orange since a blend of the two colors is visible at all times.

  Testing the Color Change Effect:

A good way to test the color change is to view the gems in a strong natural light ("daylight" flourescent bulbs are best) without influence of incandescent lighting.  Then block or eliminate your natural light source and use a focusable flashlight like a mini-Maglite to direct a strong incandescent light onto the gem.  That will show the 2 sides of color.  I do this in our office easily because we have "daylight" flourescents (about $20ea as opposed to $2ea) and simply blocking the light with my hand and shining the Maglite on the gem will show the color change.  Be aware that outdoor sunshine will completely overwhelm your flashlight so you'll need to remember that color and walk the gem inside a dark room to get the change effect.