For Gemini Lagna Emerald or Panna is recommended. Is the Lagan lord is in the second house then Emerald can be worn. As per your post, it seems that Mercury the lord of Gemini is in Cancer. Yes Emerald should be worn, even if it is in the 6th house of the Navamsha (D9) chart.
Blessings,
Navneet Khanna
Astrologer