Diablo – Difference between the achievements ‘Radiance’ and ‘Perfect As Tears’

Contrary to the current accepted answer, it appears that you do not need to listen to dialogue for a minimum of 8 seconds. I tested this today and made a quick video of my results, see it here.

In summary, you can quickly skip through all of the dialogue and still get credit toward the achievements.

Edit: I have run more tests, here is a link to a Youtube video where I skip through 18 conversations of varying lengths at full speed and get credit for all of them. It's not exactly a riveting video, but I think that it sufficiently supports my answer. Here is the video (the footage is sped up to make the video shorter).

http://d3db.com/achievement/i/perfect-as-tears

Is to do 1 perfect gem out of the 4. That's why they show you the same list as in the other achievement.

http://d3db.com/achievement/i/like-stars-in-the-sky

Is to do 1 perfect gem of every of the 4 types.

So basically, you'll get Perfect as tears first, and then do the same thing on every type of jewel. (ruby, emerald, topaz, amethyst)