I've seen other months with multiples as well. I have Feb as amethyst (DD), March as aquamarine (me) and July as Ruby (DH) and in this same set June is pearl. So when I get around to getting my ring updated, I will use pearl. I would say go with whatever "set" you've gone with before since there are lots of different interpretations.
I dont really have a set... I have a ruby ring because I hated not wearing a ring with DS so we bought a ruby one. I was searching and even the major jewelry stores don't agree. Kays has alexandrite and pearl. (and the alexandrite is VERY pretty.. a dark blue/purpley type color.. .and Zales lists Smokey quartz. SIGH.
I just ordered a new gem for my mom's grandchild necklace to add Marshall. I was given the choice of pearl or alexandrite. I went with alexandrite because my nephew is June too and so I wanted it to match the other gem already on there. The other two gems, January and February, are also translucent so I thought Pearl would look weird.
The one and only reason I'm glad Oliver held out to July 1!!! I love rubies!!! Now my kids are emerald, sapphire and ruby. I'm working on finding the ring I want. I found one that is exactly the same as my wedding band only with the birthstones instead of diamonds. But for some reason, it's silver and I want it in white gold. I can't find anyone who makes it in gold!