Geodes

I have two good-sized geodes (6" x 5" x 4") that were made into bookends. The polished surfaces (insides) appear to be coated with a clear material. Does this affect the strength of the geode? I don't know what the rock is. The interior looks a little like clear quartz, however, there is some yellow in it also. Would you recommend taking them to a dealer to help me determine what I have? Are geodes used in the same way as smaller polished crystals? Do they tend to be more powerful due to their larger size? Most of these geode surfaces are in an unpolished, natural state. Any help would be appreciated. I have just discovered how powerful crystals are in the last six months or so and am trying to learn as much as possible. Thanks.