At its very core peace is a concept of societal friendships and harmony in the absence of hostility and violence (thanks Google.com)
There is no other stone to promote Peace than the ever popular Amethyst crystal. Of course, there are other stones that promote peace such as:
Celestite Fluorite Angelite Blue Lace Agate Lepidolite.
Peace invoking notions and thoughts are what makes us see that there really is beauty in this world. Imagine a peaceful feeling you have on a quiet Sunday morning as you sit on your deck sipping your first coffee of the day surrounded by blue skies and chirping birds. This start to the day will only naturally ensure your entire day is wonderful. BUT, is this the reality?
However, if peaceful thoughts are something you struggle with, then keeping peace promoting crystals in your space may help you to get to that right frame of mind.
Having a bath with amethyst in your water is another way to help induce peace promoting thoughts. Or even popping a piece of amethyst in your drinking water and then drinking the water is another way to bring peaceful thoughts and feelings to you. Even just holding and looking at your beautiful piece of amethyst, or celestite or fluorite is enough to start the peaceful thought process.
However you look at it, there are plenty of ways to bring peace in your life through crystals and nature.