It seems we have been here before! The May gemstone is a repeat of the mineral beryl. Yes, emeralds are a different color than the March gemstone aquamarine, but mineralogically a beryl is a beryl is a beryl.
The striking green color of an emerald is caused by the introduction of just 0.1-0-5 percent chromium substituting into the aluminum spot of the beryllium silicate mineral. A small amount of vanadium is also present. It is truly amazing that just a fraction of a percentage of an element can impart such a striking difference in color. While the richness and intensity in color is paramount in valuing an emerald, the presence of visible inclusions is also important. Emeralds lacking inclusions to the eye are considered flawless and carry more value than those with visible inclusions.