In absence of a Fire Opal, I recommend a Fire Agate, a form of chalcedony. Fire Agates are formed through volcanic action when hot water, saturated with silica and iron oxide, fills cracks and bubbles in the surrounding rock. These layers cause similar diffraction to the Fire Opals and create the fiery light.
Similar to Fire Opals, Fire Agates are great activators but they may be more grounding. Agates are known for their healing and stabilizing properties and the Fire Agate is recommended as a protective talisman.
Both Fire Opals and Fire Agates have an auspicious presence and a bright vibration beneficial to the aura and chakras. Both are highly prized members of your crystal toolkit.