Pure, unsweetened chocolate, made by liquefying roasted cacao beans and containing nothing but those roasted beans, should be completely gluten-free. But pure, unsweetened chocolate also doesn't taste very good.
It's the combination of chocolate and cocoa butter (like pure chocolate, a product of the cacao beans), sugar, milk (in some cases) and other ingredients that make "chocolate" such a taste sensation. And those other ingredients are where any gluten-related problems are introduced.
It's possible even for pure, unsweetened chocolate to contain a tiny bit of gluten if it's been subjected to gluten cross-contamination in harvesting or processing (for example, if the cacao beans are processed on equipment that also processes wheat). Sadly, the chances of having your chocolate contain gluten just go up from there.
Some chocolate candy products feature gluten ingredients either in the form of wheat (most frequently in chocolate-cookie confections) or in the form of barley malt (a popular sweetener frequently used in candy).
Other chocolate candies contain no gluten ingredients, but they are subject to gluten cross-contamination because they're made on the same manufacturing equipment or in the same facilities as those gluten-containing items. In my surveys of candy manufacturers, this seems to be the biggest reason many chocolates aren't considered gluten-free.
But chocoholics shouldn't despair there are some gluten-free chocolate candies on the market.
If you're looking for a plain or flavored chocolate bar, my article on premium and gourmet gluten-free chocolate bars will help you figure out what's safe and what's not. If you want chocolates in a gift box, my article on gluten-free gift candy boxes should point you in a safe direction.
For more mainstream candy (think M&Ms and Tootsie Rolls, both of which are considered gluten-free to 20 parts per million), check out my overall gluten-free candy list, which also includes links to information on holiday-themed candies.
Although many chocolate products you see in stores won't be considered gluten-free, it's still possible to get your chocolate fix even if you're following the gluten-free diet by choosing a gluten-free chocolate from one of those lists.