Obsidian
Obsidian is a natural glass, considered to be a rock, that is formed by volcanic activity. Because of the sharpness of its edges when broken, obsidian has been used in weapons and tools like knives and arrowheads. It is often used in Native American jewelry. Sometimes, obsidian forms with gas bubble inclusions. These are known as sheen obsidians.
Where it's Found: USA, Mexico, Brazil, Ecuador, Iceland, Indonesia, Italy, and Japan.
Mohs Hardness Scale: 5-6
Onyx
Onyx is a black variety of chalcedony. It is a very versatile stone, matching just about any metal or color. When formed with layers of sard, it is called sardonyx, often featuring stripes of white, brown, or red. Sardonyx is the traditional August birthstone.
Where it's Found: All around the world, including USA, Canada, Argentina, Brazil, Uruguay, Czech Republic, France, Germany, Scotland, China, India, Indonesia, Yemen, Madagascar, Australia, and Russia.
Mohs Hardness Scale: 7
Tourmaline
One of the most versatile gemstones, tourmaline comes in just about any color, sometimes even multicolored. It also ranges from transparent to opaque. Black tourmalines are called "Schorl." Tourmaline is one of the October birthstones.
Where it's Found: All around the world, including Afghanistan, Bolivia, Brazil, Czech Republic, India, Japan, Kenya, Malawi, Madagascar, Mexico, Mozambique, Myanmar, Namibia, New Zealand, Nigeria, Russia, Scotland, Sri Lanka, Tanzania, USA, Zambia, and Zimbabwe.
Mohs Hardness Scale: 7-7.5