Not a cool rock. A boring one from next to the driveway.
I hope I’m not a bad dad. At some point my son conflated the Charlie Brown specials and thinks bad kids get a rocks instead of presents. Seeing as how he wasn’t always behaving perfectly in public during the last year (yeah, like all the other 6-year-olds were…)
I’m not going to be mean about it, all the really good stuff gets opened after the rock shows up. I’m not Frank Reynolds. But there’s gonna be a rock.
Yawn
You want to make it a holiday he’ll remember? Cat poo.
This seems like it would be a good stocking stuffer if there are other kids around.
“I got a train!”
“I got a doll!”
“I got a rock…”
I hope you are prepared for tears, and a high probability of a thoroughly ruined Christmas. 
I really would not count on giving the real presents after as certain to make everything all better.
My daughter told my husband he’s getting poo-on-a-stick for Christmas.
One year, when my kid was in late single digits, I gave him and my nephew both a gift wrapped lump of charcoal. They both had about eleventy bazillion other presents besides…
If a small rock ruins my son’s Xmas, njtt, it’s clearly not about the holiday. I’ll let you know if he still loves me this afternoon, okay?
I cry just driving by rocks on the side of the road. Rocks make everyone cry, right? They ruin not just Christmas, but every other day of the year for me.
Am I alone?
Rocks make you cry? Why?
At least be educational about it, give three: metamorphic, igneous and sedimentary.
You could put the vegemite on his tongue. Stop him swearing 
What if he treats the rock as a pet?
You’ll spoil him. When I was a kid we just got our Christmas beating, before going to work to the mines, and we liked it!
When I grew up, only rich people could afford to give their children rocks.
We were so poor…
…that all my dad gave me for Christmas was a pit thread, before beating me to death with his belt. And I liked it!
We are moving into kinky territory here.
Only if the rock has a whip.
Rocks. What a lucky kid. One year all I got was 3 pebbles and 7 grains of sand.
Oh-la-di-da. Mister Moneybags had grains of actual sand! We used to dream of getting sand! My parents would save up all year just so we could have some dust for Christmas!
One Christmas I got a single molecule. And not even a good molecule. I think it was Iron.