Obsidian is formed when lava cools very rapidly , not allowing time for crystals to form , hence it resembles very smooth black glass. If it cools slightly more slowly and the mineral cristobalite is present, then white snowflake-like specks appear in the Obsidian , creating this magical effect.
It was used in ancient times as a cutting tool as it can be split to create very sharp edges and is still used as a surgical tool . As the gemstone has a high reflective index larger pieces were also used as mirrors.
Snowflake Obsidian is used as a protective amulet , it may absorb negativity and stress.