Stone Wheelies - 30mm Offset Mahogany Obsidian Brown - 20" to 24" Adjustable Soft Black Poly Woven Washable Cord - Natural Crystal Gemstone Collectibles Carved Necklace Handmade Charm Stone Type: Mahogany Obsidian. Color: Red Brown Black. Approx. 30mm Diameter Description: Donut Shaped Gemstone with 20" Adjustable Cord Necklace. Gemstone Symbology: Obsidian # Sincerity, Honesty, Truth, and Strength. Sagittarius Birthstone (November 22nd # December 21st). A form of naturally black volcanic glass, commonly considered a gemstone despite having no defining crystalline growth/structure. Obsidian is formed when chemically favorable lava flows cool above ground. Includes Adjustable 20" Black Necklace Cord as Pictured Stones may vary slightly in size, color, pattern, or shape. The item sent to you will be similar to the one picture first.