Using a rock to store excess anger, stress
Stress that hits the digestive, stomach, head, shoulders, and neck areas usually has to do with anger and the inability to express it for whatever reason.
Sometimes breaking your reaction patterns with people in your life creates a place where anger at the previous position tends to ball up. That creates a pocket where you would normally have stretched out anger at someone, but now you find that it is still being created but has no place to go.
You're seeing things that you "know" to be true, but the masks people live through are angering you as well. The only solution is detachment and learning to ignore what you see, sometimes.
The old shamans used to store their souls in power objects like that, and some of them are still around. I'd advise leaving them alone.
That seems the key, but how?
Get a small rock; one you can easily fit in a pocket, just a common old rock that will work with you. Ask it if it will receive your anger and store it for you. Take this rock and bury it in sea salt for one night. Then, take it out and rinse it off. Put it in the bottom of a dry pot and heat it on the stove on a low heat for about twenty minutes. Once it has cooled, hold it in your hand and concentrate on opening it for your energy. You open the rock by using your breath. Put it to your lips and draw air by inhaling, across it and into your lungs. Feel the air cross the surface of the rock and feel it's energy as it enters with the breath. Do that until you feel comfortable that you have the feel of it and the rock is open. It will just strike you as a feeling. Now exhale onto your rock slowly, do that several times until you "feel" it take the hook of your energy. Now, you are connected. Don't let anyone else handle or touch your rock. Put it in your pocket when you go to work. Feel it in your pocket and feel a current running down your arm to the rock from wherever the stress in your body seems to be settling the most or the anger. As you use the rock for its new purpose, take note of the feelings it produces, in there you will find detachment and how it strikes you. The recapitulation as a process will eventually bring you to the same point, but it will be more accessible to you intellectually, and therefore you will have a better understanding on an emotional level.
The only solution is detachment and learning to ignore what you see, sometimes.
Now, anytime you begin to feel anger, stress, or that you need more grounding, use your rock. You can periodically allow your rock to 'discharge' its energy by placing it on the earth overnight, then collect it the next day. When you feel the rock telling you it doesn't want to do this anymore, take it somewhere special, a place a rock would like to be, and give it the dirt nap. :) It would be bad for someone else if they picked it up and carried it around with them.
There you have your first lesson in working with the mineral allies for your and their benefit. The rock gets a nice new home and has no problem grounding out the energy, and you get to release a lot of anger/stress/whatever. Once it hits the earth again, the energy will be dispersed.
Is there meaning in the ritual of heating it, etc?
Yes, you are clearing it with the salt and lining up its energy with a gentle heat.
Why would the rock want your anger?
Some will take it as a service; they will work with you.
Is any particular type of stone more amenable than any other?
Some are more so than others, but in general, I think it's best just to find one you like. One that is closer to your energy . The old shamans used to store their souls in power objects like that, and some of them are still around. I'd advise leaving them alone.