"My wife is my rock" = my wife is very supportive of everything I do

As a metaphor, a rock serves as an anchor, an center of stability that prevents a person from drifting away and straying from their core beliefs. It also brings the image of someone with a definite purpose in their life that does not get distracted from that purpose. By saying his wife is "his" rock, he brings the image of the anchor for his life, the first image I mentioned.

A rock is something you bash you head against with no effect on the rock but with great damage to your head...just like a wife.

Though you will get weird looks if you say this. Just reading that I gave my computer screen a weird look :) It's very uncommon to hear or read that.

Originally it was Scottish, the Stone of Scone, and rested at the king's feet and was part of Celtic religion. Each ancient Celtic family was said to have a "home stone" with that family's legacy carved on it by each present master of the house and passed down from generation to generation; it's what made a house a home. Lose that stone and you have no home. A rock is a stone.