In a drawstring bag, in the attic, under the insulation.

Conversely, in a glass, sealable bowl, in the sump pump pit.

In the fireplace

I've got a dead birch tree out back. Drill a tiny hole. Maybe a bit of metal over top so a woodpecker doesn't steal it, lol.

How big is this diamond? If it was small, it is much easier to hide anywhere. I would hide it in the kitchen.

In my bladder

Outside in the garden.

In the cat's litter box

In the ice cubes!

In the dog

In the clock.

I'd smash up some glass and put it in a plastic bag under the doormat so if they tortured me I'd have a story to give them if I was tortured. Then I would put it in a tinfoil wrap with a couple small pebbles so it would get caught in the p trap under the sink and drop it into the drain. Then put the stopper in, fill the sink with some kitchen grease dishes and soap water. Pour out my whiskey bottle most of the way and put some sedatives in that. Then go find my bag of big zip ties, plastic drop cloth, hefty lawn bags, shovel, meat cleaver and bone saw and put them in the trunk of my car.

Hypothetically, of course. If I had a diamond, which I do not.

Dirty laundry

Down a drain.