Gamera vs. Barugon

Set after the events of Gamera, a meteorite collides with the rocket transporting the creature to Mars. Now free, Gamera returns to Earth. At the same time, three mercenaries are sent by Kano, a WWII veteran, to an island in the South Pacific to retrieve a huge opal he once found and hid in a cave. Despite warnings from the local villagers, the trio finds and locates the opal.
The second man, Keisuke, Kano's younger brother, is betrayed by his fellow expeditioner Onodera back to Japan, Onodera accidentally leaves the opal exposed to infrared light, incubates the opal, and a monster Barugon eventually hatches. Growing to enormous size very quickly, the Japanese defense ministry suggests a plan to take on Gamera.