Birthstones is an interesting idea when it comes to buying a gemstone for someone.
Here's a list of gemstones corresponding to the birth month.
January - Garnet
February - Amethyst
March - Aquamarine
April - Diamond
May - Emerald
June - Pearl / Alexandrite
July - Ruby
August - Peridot
September - Sapphire
October - Opal / Tourmaline
November - Topaz / Citrine
December - Tanzanite / Zircon / Turquoise
Pictures: TKBESPOKE - Kristina Lim