The Boss Stone is a treasure in Pikmin 2 that is actually a Shogi King tile. It is found in the 11th sublevel of the Dream Den. It is the 142th treasure on the List of Pikmin 2 treasures in the NTSC version and the 140th in the PAL version.
Captain Olimar's Journal
"My long journey has led me to this compelling stone. During my quest, I've narrowly escaped being crushed, chewed, ingested, and scorched countless times. But somehow I persevered, and this stone reminds me of that. It's like it has a personality of its own. I'd like to take it back with me and place it in my garden to commemorate this expedition."
"For those who care for others... For those who hail subordinates and send them into the unknown... Only these few will appreciate this fine, distinctive stone... (Did I just describe myself?)"