Which of the following gems is usually considered eye-clean?

Spodumene is often considered eye-clean because, when well-cut, it typically exhibits very few visible inclusions to the naked eye. This clarity makes it appealing to gem buyers, as most natural gems possess some level of inclusions, but spodumene stands out with its potential for clear specimens.

In the context of the other options: Rubies frequently contain visible inclusions, such as rutile needles or other crystal fragments, which can affect their overall clarity. Emeralds are known for their characteristic inclusions, often referred to as "jardin," which are frequently visible and accepted within the industry as part of their charm. Diamonds, while they can also be found in eye-clean qualities, often contain small inclusions that can be detected without magnification, making their clarity grades essential in determining their value. Spodumene's typical lack of visible inclusions sets it apart as a gem that is more likely to appear eye-clean compared to the others listed.