(1) Oxygen can be used for human breathing and is often used in medical emergencies, so the answer is: oxygen;
(2) Calcium carbonate contains calcium and is a commonly used calcium supplement. So the answer is: calcium carbonate;
(3) The material that can be used to make a glass knife must have high hardness. Diamond is one of the hardest substances naturally occurring; it is suitable for making a glass knife; therefore The answer is: diamond;
(4) Carbon dioxide does not support combustion and can be used to extinguish fires, so the answer is: carbon dioxide;
(5) Petroleum can be refined into gasoline and kerosene, so the answer is: petroleum;
(6) Ammonium bicarbonate is a commonly used nitrogen fertilizer, so the answer is: ammonium bicarbonate.