As far as i've seen in game, when testing out these things, im led to believe that it's actually not a typo.
The stones that has a chance to plant to cause dmg, is :
10% chance to plant cause dmg 150;
Means that you have 10% chance while fighting a plant, to hit 150 REGULAR dmg extra.
This will be added to your damage, and not shown.
If you really wanna test it out, get a white 2h sword with 6 slots and fill em with the stones that adds, say, Dmg to Beast, and find an area than has beasts in it. Would be sweet if you had a lower level char to test with.
Now start damage dealing. Make sure that you have at least 100 dmg+ to the element.
You will soon see that it actually does what it says.
The Dark Damage ALSO applies to the regular damage. However on this, you can see an animation when you do so.
Try using a flaming fang sword.. It will not show any fire damage in the dmg taken/dealt info, but sometimes it will just hit those 200-300 harder and give a fiery animation to the hit.