Spectacles

[Rusvul]

I think rock crystal, specifically Quartz or Mica. they both are very clear when pure, and neither are expensive. As Axehound said, you can find quartz in your backyard, depending on where you live. I think either Quartz, Mica, or glass. DEFINITELY not diamond. At least, that's my opinion.

[321tumbler]

I agree. Diamond is precious so it wouldn't be used for spectacles.

[Redwaller]

Even if they had diamonds, it would be too hard to make spectacles of them. They're probably made out of glass.