I suppose it depends upon whether they will be in jewelry of some-type. You may be able to find some inspiration in occult for some of it. Playing cards, for example, were used for fortune telling. Depending upon the school of magic you're going something such as : Clubs ( Air / Spring), Spades ( Fire / Swords), Hearts ( Water / Autumn), and Diamonds ( Earth / Winter) may work.
Or perhaps Spring would be a marquis cut ( maybe yellow , chartreuse, or Alice blue - blue agate maybe), Summer a trilliant cut ( deep pine green or sunset orange maybe--- even bloodstone). Autumn a pear or drop cut (the color of autumn leaves, perhaps a chocolate colored gem, orange, or red - maybe even a tiger's eye). Winter a square or round cut ( clear or deep blue or maybe something like a moonstone). Or Winter could even be something of a cabochon that's not been worked much - a rough stone in the raw.
You could perhaps even use gemstones that already correspond with those months and astrological signs.