This quickie will show you How to Draw a Rock using Inkscape.
Start out by using the Pen Tool.
Draw a random shape to begin your Rock, keep the nodes Sharp so it looks blocky.
Give your Rock a nice gray color to start with.
Create a few Nodes positioned together.
Grab a middle node and Drag it inwards.
This will indicate Chips and Breaks in your rock.
Create more of these chips and breaks throughout the shape.
Now we need to add Shading in the area inside the chips.
Take your Pen Tool and draw a shape that fits the area, don't mind if it goes over a bit.
You can even create more than one Shape to fill in the chips, just makes sure they are both darker shades than the Rock color.
Then make sure to Lower the shapes behind the rock.
Continue to do this for the rest of your Chips and Breaks as you see fit.
Using your Pen Tool again, draw another Shape ontop the original, but along the bottom of it.
Make sure to give it a Shade that's a darker color.
To indicate Cracks in the rock, use your Pen Tool to draw a shape similar to the one shown below.
Make sure the shape matches the Edge of the rock, where the two shades of color meet.
Draw one last shape at the bottom of your last shape.
Once again, give it a darker Shade of color.
Create patches of Dark shapes to indicate little to no light reaching the area.
Do this a few times as necessary.
And you'll have yourself a nice detailed Rock!