Obsidian is dense volcanic glass
usually black in color. Inclusions and bubbles add color and patterns to some
types of obsidian. It is often formed in rhyolite lava flows when the lava cools
quickly enough to prevent crystal formation. Glass has no regular structure
and fractures in round smooth sections. These can form an extremely sharp edge
and obsidian was used by many cultures for knives and arrowheads. Today obsidian
bladesare used in heart
surgery, and have a cutting edge much sharper than surgical steel scalpels.
Onyx is a cryptocrystalline form of quartz. Bands
are usually white, tan and brown but can be a wide variety of colors. Sardonyx
has red bands of color. Black Onyx is less common than banded Onyx. Used for
cabochons, beads and cameos. Onyx has been used by a variety of Ancient cultures
and civilizations.