There are different types of plastic used for lenses, the toughest is polycarbonate, as used in kids' glasses, safety glasses and proper cycling glasses. Polycarbonate has a refractive index of 1.58, which is useful to remember as the brochures often use cryptic trade names. It is also possible to get impact-resistant lenses that are made of other materials in a sandwiched construction, but solid polycarbonate is the most common.
If, like me, you are quite short-sighted, prescription wrap-around lenses end up heavy and thick around the edges. The thick edges distort vision and can be disorientating if you swap between wrap-arounds and normal-shaped glasses. For this reason, I wear normal glasses that are large enough to cover my eye when my head is tilted slightly downwards, as it is when riding. Good coverage is important for blocking wind, sun and flying stones.
Proper cycling glasses have excellent coverage and peripheral vision, better nose/ear grips that don't slide around as much, and many are specifically tested for a defined level of protection against flying stones. Normal glasses with polycarbonate lenses are far better than nothing against flying stones, but the degree of protection is indeterminate, as the lens may be thinner, even if it is made of polycarbonate.
Some normal glasses frames have very thick arms which can block peripheral vision, so avoid these.
In terms of tint (for bright daytime use only!) my preference would be for polarized (which is NOT the same as Polaroid, which is a brand name) - this is technically not a tint, but it does cut out half the light, which, for me, is sufficiently dark for comfort on bright days (in Britain, at least) without making me completely blind if my ride is unexpectedly delayed into the hours of darkness. More importantly, polarized lenses reduce reflections, for cycling I find this advantageous for (1) spotting potholes lurking under puddles of water, and (2) seeing through car rear windows. Unfortunately, you can't get polarized lenses that aren't inherently tinted.
For long rides that extend from day to night, I take 2 pairs of prescription glasses with me, one is clear and the other polarized/tinted, but both are similar in lens size and shape so my eyes don't have to adapt to differences in distortion.
Incidently, I don't wear polarized glasses when driving in the rain, as I want to be able to see and avoid the puddles on the road and am less concerned about the potholes that may be lurking beneath them.
