What is the number of different magic properties that a hellfire ring can have? [duplicate]

All items crafted by the blacksmith have a number of random magic properties. However, I've tried to create a helm that provides a bonus to Dexterity and failed after 15 tries.

Are the properties randomly selected from a universal set or are they selected from a set specific to the item being generated?

There are more options now. Starting in patch 1.0.5, Blizzard has been adding recipes that guarantee a particular stat. First came the Hellfire Rings (see here for instructions).

Patch 1.0.7 added five new recipes, each of which comes in four variants. Each variant comes with a specific primary stat: Dexterity, Intelligence, Strength, or Vitality. For the Jeweler, there are four Amulet variants.

That's currently twenty-four recipes with a guaranteed roll of some stat.

The random properties available are dependent on the specific piece of gear you're generating. Boots are more likely to increase movement speed, for example.

It's a little bit early to have an exhaustive list of what kinds of bonuses are available on which items at this point, but if you're not getting Dexterity on the piece of gear you're trying, you might have more luck on another piece.