Obsidian.

Obsidian is volcanic glass, formed when felsic lava cools too quickly for crystals to grow. The result is amorphous silica with a conchoidal fracture sharp enough that flaked obsidian blades have been used in archaeological tool kits and in some modern surgical scalpels. The major varieties differ by inclusion: snowflake obsidian carries cristobalite spherulites, mahogany obsidian shows iron oxide bands, rainbow obsidian holds magnetite nano-crystals that produce sheen, and Apache tears are the small naturally weathered nodules of translucent black glass.

Across cultures obsidian served two parallel uses. As a tool material it edged Aztec macuahuitl blades, Anatolian neolithic knives, and Pacific Northwest fishing implements. As a scrying surface it appeared in John Dee's sixteenth-century Tudor cabinet (the polished Mexican obsidian mirror now in the British Museum) and in Mexica divinatory tradition long before. Both uses share the same basic property: obsidian shows a clean reflective truth without much filter.

In modern crystal practice obsidian sits with the root chakra and is treated as one of the more honest stones. It is the piece people choose when something needs facing rather than soothing. A polished sphere or palm stone is the most common form, and the weight in the hand is part of why it works for grounding. Avoid steam cleaners, which can fracture the glass through thermal shock, and handle freshly napped or chipped pieces with care because the edges are genuinely sharp.