Your stone is Black Obsidian, the stone of clarity.
Unlike the other stones, Black Obsidian naturally retains warmth and is great to use on congested, blemish prone skin. The energy of the stone draws out tension and promotes the removal of blockages. The warmth of the obsidian encourages stimulation of the lymphatic system and circulation. This energizing of the skin reduces toxins and promotes clearer, brighter skin. To warm our Black Obsidian tools, we recommend placing them in a glass of warm water.