The crown chakra is the chakra who makes us the best connected to God and makes
us capable to be enlightened and gives us knowledge. You can open or close this chakra during a meditation by just visualising this chakra like a flower in the colour of this chakra wich can be white, gold or light purple depends on you. With this chakra, we can connect with not only God and angels but also with our Higher self and spirit guides.  When it is open you are aware of your surroundings and understand everything in life or accept things better, and you become more self-aware.

You know if the chakra is open by thinking if you are stable in life and more conscious of life and aware.  

Make sure you are good grounded too otherwise you will fly high, can be nice but for our daily life it will not help, daily things like your job, for example will be different and things will not work. So make sure you are good grounded too.    

Stones by this chakra : Amethyst, Mountain crystal, Moon stone and Dessert rose of course there are a  few more like Selenite but those crystals are more used by this chakra.