You are asking a great question, however it is always better if you put your birth details as well so that experts can check if Red coral is recommended. Generally speaking, it is not that if Mars is in Gemini Red coral cannot be worn. It is the nature of Mars that needs to be checked. If Mars is benefic or malefic. If Mars is benefic then Red Coral can always be worn.
Blessings,
Navneet Khanna
Astrologer