calcified stones?

I"m a bit confused about what calcified stones are. The first time I did a flush several years ago, the stones that came out were hard, dark, and had flat sides. They looked like they had been packed together and flattened each other out. I was a bit surprised to see the green stones that I passed a couple weeks ago as I expected to see the same dark kind, although I did pass a few of those. Are the dark ones the calcified stones and the green the cholesteral stones?
Thanks for info anyone..
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