When describing stone beads, you mentioned "grades". What did you mean? Gemstone and stone beads grading is less formal than gemstone grading as a whole. Still, there are some generally accepted rules that determine which grade is assigned to a strand. Grade is assigned based on a letter with "A" being best and "D" being worst. Note that "D" might be just what you want for a specific project, so don't dismiss them as worthless. Sometimes you might see gem grades of "AAA" and "AA". Those are "real" gem grades with "AAA" being near perfect. It's rare for such good quality stones to be cut into beads. They are worth more as faceted gems. The overall quality of the stones in the strand is usually derived from a combination of the classic gemstone qualities of color, cut and clarity. If the strand fails one or more of the tests for these attributes, it gets bumped down a grade or three.
Desirable color is specific to the type of stone in question. Sapphires are more valuable when they're blue, even though some of the yellow ones are very pretty. Generally stronger colors are more desirable than paler colors. Even a slightly muddy dark blue sapphire is worth more than the clearest pale blue. Pearls are an exception. The closer a pearl is to white, the more valuable. Come to think of it, some naturally colored pearls are an exception. A natural black pearl is a lovely charcoal grey and very valuable. Even color with no light and dark zones is more desirable, except for stones for which a color change is actually more valuable. Watermelon tourmaline shades from white to green to clear to pink. If the color has been enhanced, by dying or heat treating, then it should be dropped a grade. Sometimes this rule is ignored when all the stones of that type are enhanced and the change is permanent.
Cut is a combination of shape, size and finish. Round beads should be perfectly round to the eye. Faceted beads should have facets that are evenly spaced and sized. All of the beads in the strand should be the same size. Except graduated beads. They should step down in even increments. The beads should have a bright shiny finish. Except, of course, when the stones are deliberately matte. Pearls have a quality called luster. Luster is the pearl's ability to reflect and diffuse light, it's pearliness. The greater the luster, the more valuable the pearl.
Clarity in beads is usually applied as a lack of flaws. While deeper colors are more desirable, transparent stones should also be clear and sparkle. Clarity can be a function of how well the bead was cut. Poorly cut stones are often dead looking. Opaque stones should not have any cracks or undesirable inclusions. Some inclusions are desirable, good quality lapis lazuli usually has veining of golden pyrite and white calcite.
Now that you know the basics, how do you acquire practical knowledge? Go to gem shows, rock shops and bead stores. Notice which strands are most expensive. In a reputable shop, the more expensive stones are usually of a rare material or better cut. Ask the sales clerk why strand "X" is worth more the stand "Y". Then touch the stones. If you could look over my shoulder when I'm buying beads, you'd occasionally see me pick up a likely looking strand, only to immediately replace it on the table. That usually means one of two things, they were of poor quality or so good that I probably could not afford them. If it's the former I may handle some other strands before deciding that the merchant is wasting my time. If it's the later I'll ask the price, just to see if they got a good deal on them and are willing to pass it along. So go out, look, touch, and ask questions.