Which lens material is the least scratch resistant among those mentioned?

CR-39 lens material is the least scratch resistant among the options listed. This is primarily due to its chemical composition and the manufacturing process. CR-39 is a type of plastic that is known for its lightweight and optical clarity; however, it lacks the advanced coatings and durable characteristics found in other materials, like polycarbonate or Trivex, which are often treated to enhance scratch resistance.

In contrast, polycarbonate and Trivex have inherently higher impact resistance and often come with built-in scratch-resistant coatings that improve their durability. The 1.67 high-index lenses, while thinner and designed for those with stronger prescriptions, also typically offer better scratch resistance compared to CR-39 because they can be treated similarly to polycarbonate and Trivex lenses.

Therefore, if scratch resistance is a primary concern, it's important to consider lens materials that offer enhanced coatings and properties, like polycarbonate or Trivex, rather than CR-39.