Also known as heliotrope, there's a reason this crystal is better known as bloodstone -- because since ancient times it has been used to help clean and balance the blood. As a green crystal it helps channel the earth's energy of growth, replenishment, healing and vitality, and so it is also useful for helping to balance moods and heal wounds.
It can, in fact, be used as protection against injuries and strains (although I suggest you still lift with your knees and not your back!), and can be used to support you in times of stress and illness.
2cm - 3cm Average Size
Your tumblestone will be intuitively chosen by me for you.