I've used found objects as a starting point for these sketches.
A friend set the challenge above which was a small stick.
The one below was rectangular shape with a corner, cannot remember exactly what.
I did these a while ago, but now I try to remember to draw around the item as a reference, like in the flower picture below.
another one set by a friend, this time a pebble
I created both of these using the pebble.