Yes, the everstone.
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Causes Evolution; Fire Stone Leaf Stone Water Stone Thunder Stone Moon Stone Sun Stone Dawn Stone Shiny Stone Dusk Stone Prevents Evoltion; Everstone
The evolution stones in Diamond/Pearl/Platinum are the water stone, fire stone, thunder stone, moon stone, leaf stone, sun stone, dusk stone, shiny stone, and dawn stone. (Sorry if I missed one) And even though this is the opposite answer, the stone to keep Pokemon from evolving (with or without evolution stones) is the everstone. Hope this helps! :)
I am not sure but I think it is hidden somewhere
You level it up/you evolution stone.
You can't find it in the wild because its a stone evolution.