(Chart by kind permission of the International Gem Society/Ganoksin).
Several months have alternative birthstones. For example, June - an alternative stone is the pearl, which is not really a gemstone because it is organic matter rather than a mineral. October has the opal as an alternative, and November has citrine. December has two alternative gemstones: turquoise and tanzanite.
For more information on birthstones, see our blog on birthstones.