While buying a geode is always a guessing game, there are a few simple tricks you can use to increase your odds of finding one that is filled with beautiful crystals. Rather than randomly hunting and picking through a pile of geodes, use a more careful method of elimination. Since geodes that contain crystals are usually lighter than those that contain solid agate--and the color of the exterior sometimes corresponds to the color of the interior--you can eliminate those geodes that are unlikely to contain what you want and increase your chances of going home with a geode you love.
Pick up several different geodes, comparing their weights and choosing the lightest one.
Continue comparing weights, setting aside any that feel somewhat light for their size until you've set aside about a dozen lighter geodes.
Weigh the small pile of geodes you have set aside, eliminating the ones that are heaviest for their size.
Inspect the remaining geodes for size and shape, eliminating any that are not the size you are looking for or that are not an eye-pleasing shape. They do not necessarily need to be perfectly round, but if you dislike the shape, eliminate that geode.
Set aside geodes that are extraordinarily light for their size.
Inspect the light geodes for signs that they may contain mud rather than crystals. Although there are not always obvious signs, mudballs will often have brown or black streaks on the exterior and may have pieces of the exterior chipped away, revealing an ugly brown interior. Eliminate any geodes that are likely to be mudballs.
Examine the exteriors of the remaining geodes. If the exterior is completely white, without a blue or black undertone, the geode is likely to contain white quartz crystals or light-colored banded agate. If it has black, blue or purple undertones, it may contain amethyst or other colored quartz crystals or blue banded agate. Keep the geodes with exteriors that match what you hope to find inside.
Check the exterior for any chips or cracks that reveal the interior. Although it's not common, some geodes will have small openings that will reveal their inner contents. If you see a hole or crack that reveals something you like, hang on to that geode.
Weigh the remaining geodes, choosing the ones that are lightest.