Obsidian (volcanic glass) is black and sometimes translucent. There are many varieties of obsidian including mahogany, snowflake, rainbow, fire, gold sheen, and probably a few others I don't know about (yet.)

There is one place known as Glass Buttes, Oregon that I'd like to visit some day. I am told it is a vast deposit of many types of obsidian.

The mahogany variety has streaks of brown in stripes and swirls. I don't know where it comes from, but it polishes nicely. The few pieces I have came from various grab bags and assorted bucket sales.

Here's more information about different types of obsidian.