The most common chromium ore is \( \text{Cr}_2\text{O}_3 \), also known as chromium(III) oxide. It is the primary source of chromium for industrial purposes. Other chromium compounds, such as \( \text{Cr}_2\text{O}_4 \) and \( \text{CrO}_2 \), exist but are not as commonly used as ores for chromium extraction.
Thus, the correct answer is \( \text{Cr_2\text{O}_3} \).