Have a crystal that after meditation changed to dark grey. I know that I have a lot of anger is this how the crystal is helping? Please don't ridicule...I have seen posts where people are mean. It's an honest and frankly scared question.
To be honest I doubt it was due to some anger or the meditation, always try to rule out other possibilities before saying something supernatural happened.
It is a common belief that crystals and gems vibrate with their own, natural energies. They are believed to be able to store it, channel it, and balance it. That said, while your energy may have interacted with that of your crystal, I doubt that it was the reason it changed color.
Denae, the others have given you some good answers on this topic. It is quite common for crystals to change color for a number of reasons. Sunlight, contact with water, and other environmental factors can all play a part in coloration changes within a crystal.
I've heard and seen that when you get a crystal, it will actually change it's form into one that will be most helpful to you. And I know how confusing that last sentence may sound so please allow me to explain. I use to have a white alabaster crystal that was palm sized and shaped like a donkey. I had dropped it(not on purpose) onto the floor and one of it's ears came off. I told it I was very sorry and it responded to me through thought saying that it (referring to the crystal) did it to itself, because in this form (only having one ear instead of two) would be better for me because this way that one ear has more energy. Because crystals can talk to you through thought, also I have a clear quartz crystal and mine has only turned "darker" when it has to be charged and cleansed and retuned. Also a crystal's color may change when it needs to be recharged and cleansed. So may I suggest you place it on your window sill (I charge my crystals at night and then I take them off before I head off for the day) What I say is " I call upon Hecate, Cerridwen (pronounced KARE-id-ooín )and the Great Divine, And I ask that you cleanse this (name of crystal or if I'm doing this for more than one then I just say "cleanse this crystals that I have placed on the window sill) of all outside energy, I also ask that you please charge these crystals (or the name of the crystal if it's only one or two) with your divine energy, and please retune these crystals energies to the earth, and please allow all of these crystals to soak up the powerful energy of the sun and the moon so that they/it can provide the service that nature intended for it, thank you so much, Hecate , Cerridwen (pronounced KARE-id-ooín) and the Great Divine, thank you so much, so bless it be" Then I bow as a sign that I'm showing respect. I hope this is helpful to you.