These aren't hard rules. There are options. If you don't like your stone there are always alternatives.
April - sorry I don't currently offer white diamonds, so I go for the alternatives of white topaz, or cz instead.
June - the traditional birthstone is pearl, which isn't hardwearing enough in my view to be used in everyday jewellery, so I use the alternative stone, which is moonstone. It's more hardwearing and is absolutely beautiful!
October - there are two stones for October, opal or tourmaline. I use opal the most - but the choice is yours!
November - the traditional stone is yellow topaz, but this is very hard to come by these days so I use the alternative which is citrine. Some people prefer to use a different topaz instead - there are lots of blue topaz to choose from.
December - there are a few stones that represent December. The key one is Turquoise, but the clear stones are tanzanite, london blue topaz, or swiss blue topaz.
If you're having trouble imagining how your stones will look together, please get in touch and I will hopefully have the stones you're interested in here, and can send you a picture or video. Have a peek in my gallery page as well, as the stones you require may be featured there!