It became popular during the reign of Queen Victoria, when the Queen wore Whitby jet as part of her mourning dress.
During the peak of its popularity between 1860 to 1890, more affordable and faster reproductions of authentic jet were used as it became the height of fashion.
The most common is French jet, which is dark colored antique glass. Also common is jet made from pressed horn and wood.
So how do you know you're getting the real deal? Well, first you need to be prepared to pay for it. Most authentic jet, also called Whitby jet from Whitby, England is 182 million years old. And as with any vintage jewelry one would be lucky to find Whitby jet from the 1800s in good condition. The initial difference is jewelry made of other materials will be cool to the touch and heavier. Though this is not a fail-safe method. When rubbed against unglazed porcelain, true jet will leave a chocolate brown streak... Hmph, try that at the next antique store you're at. ;o)
French jet and other alternatives will be much easier to find, more wearable, and will still add an exciting, dark Victorian edge to your jewelry collection.
Mostly my searching takes place on Etsy, and if it's truly real jet you're craving you can check out this site, or this one. Beautiful!