These birthstones were chosen based on their availability and affordability. Here are the modern birthstones associated with each month: January: Garnet February: Amethyst March: Aquamarine April: Diamond May: Emerald June: Alexandrite or Pearl July: Ruby August: Peridot September: Sapphire October: Tourmaline or Opal November: Topaz or Citrine December: Tanzanite, Zircon, or Turquoise