plum, plumb

Plum or plum means the small summer fruit you see in the supermarket--
She found a recipe for plum cake.
Don't eat those plums; they're not ripe.
Prunes are dried plums, just as raisins are dried grapes.
Plum also means a deep purple color, even though plums may be red or yellow as well as purple--
That plum sweater is her favorite.
We are looking for plum paint for the house trim.

Plumb or plumb ( rhymes with plum ) means digging to the bottom of something, figuratively--
He was determined to plumb the well of philosophy in its entirety.
A good detective can plumb the darkest motives of human beings.
Plumb also means a lead weight used on the end of a string to mark a straight line on a wall, or to measure how deep something is--
He used a plumb bob to make sure the wallpaper was straight.
He used a plumb line to find out the depth of the well.

Now that you know that, you can say--
"A plum pit is not heavy enough to use as a plumb bob."

Note--"plum" may mean a prize, or an unexpected goodie--
"off of the tree of life I just picked me a plum..."
Note #2--"plumb" is an old slang expression for completely--
It plumb slipped my mind.
Note # 3--a "plumber" fixes "plumbing"--sinks, toilets, pipes and drains--
The toilet is leaking; call the plumber.