Final answer:
Chromium (Cr³⁺) ions are necessary for beryl to become an emerald, giving it its distinctive green color. Other cations like iron, vanadium, and manganese do not have the same effect.
Step-by-step explanation:
The student has asked which cation, if not present, will always prohibit the formation of Emeralds. The answer is that the presence of Chromium (Cr³⁺) ions is essential for the formation of emeralds. The mineral beryl becomes green and known as an emerald when approximately 1% of its Al³⁺ ions are replaced with Cr³⁺ ions. Without Chromium ions, the beryl cannot exhibit the characteristic green color that defines an emerald. Other cations such as Iron (Fe), Vanadium (V), and Manganese (Mn) do not have this critical influence on the green coloration of emeralds.