Obsidian

If you frequent certain areas of social media, or visit new age mineral shops, you may have seen stuff touted as obsidian that is nothing but colored glass.

Real obsidian is volcanic glass. It's formed from volcanic eruptions when lava (molten rock) cools so quickly minerals don't have time to nucleate and grow. Most obsidian is black and glassy with distinctive conchoidal fracture.

Conchoidal fracture can be seen in the sample above and is the curved, shell-like way obsidian (and other glass) breaks leaving sharp edges. Native people around the world took advantage of this fracture pattern to use obsidian to make knives and spear tips.

Some obsidian has inclusions of a white mineral called cristobalite (a form of quartz) leading it to be called snowflake obsidian which is very pretty when tumbled or carved.

Mahogany obsidian has a brown coloration from some trapped iron oxide in the rock.

There are also some rare varieties of obsidian (sheen, rainbow, or fire obsidian) that show a metallic sheen or colorful iridescence. This is usually from microscopic gas bubbles or different mineral inclusions that reflect various wavelengths of light.

When visiting metaphysical shops that sell crystals, as I'm wont to do out of curiosity (I usually don't buy from them because I can go to rock and mineral shows and buy genuine samples at a fraction of the price), I have started seeing a lot of colored glass being sold as "obsidian". Here are some examples from Etsy.

While marketed as obsidian, and sometimes priced at hundreds of dollars, these are really just hunks of colored glass. They're usually formed as a by-product of steel manufacturing and found in industrial dumps. One variety that I know of is artificially manufactured from Mt St Helens ash (Helenite) and sold as natural. It about as natural as your kitchen window.

These are often marketed as Andara crystal, blue, green, or red obsidian, Gaia obsidian, Aqua Lemuria, and other made-up names with sometimes outrageous magical claims associated. Break a green beer bottle if you want some green glass - it will have about the same healing energy as these "crystals" and you can drink the beer beforehand.