Gold has been recognized and highly-valued since prehistoric times. It might have been the first metal used by humans and was valued for both ornamentation and rituals. Gold is the most malleable and ductile metal; a single gram could be beaten into a sheet of one square meter, or an ounce into 300 square feet. Gold leaf can be flattened thin enough to become translucent. The transmitted light appears greenish blue, as gold strongly reflects yellow and red. It readily forms alloys with many other metals. Gold is an excellent conductor of heat and electricity, and is not affected by air and most reagents. Heat, oxygen, moisture, and most corrosive agents have very little chemical effect on gold, making it very well-suited for use in coins and jewelry. Pure gold is so soft for ordinary use and is hardened by alloying with copper or other base metals. The gold content of gold alloys is measured in carats (k), pure gold being designated as 24k.