The term 'certain birthstone' can refer to two different gemstones: OPAL and RUBY. OPAL is a hydrated amorphous form of silica, which is composed of silicon dioxide and water. It is known for its unique play of color, which is caused by the diffraction of light passing through the stone's microstructure. RUBY is a variety of the mineral corundum, which is composed of aluminum oxide. It is known for its deep red color, which is caused by trace amounts of chromium in the stone. Both OPAL and RUBY are popular choices for birthstones.