FMF - Friends of Minerals Forum, discussion and message board

[davidcalhoun]

newbie. first post. Question- To tell if that round rock is actually a hollow geode, or solid, has anyone ever tried soaking it in water, with something like an inverted glass jar over it, to see if bubbles come out of the rock maybe telling us that it is hollow (and air-filled)? thanks for any answers. David

[Les Presmyk]

Chances are if you put your round rock into water, there will be no bubbles coming out of it. The easiest way to tell if it is a geode is to split it in half. This can be done three ways. First, find someone with a cast iron/clay pipe cutter. They look like a bolt cutter but with a chain comprised of tungsten carbide wheels rather than jaws. Plumbers should have one (whether they will let you use it to split a rock is another problem). Just put the rock in the chain and split it. Or, go to a gem & mineral show and hopefully there will be someone selling and splitting geodes. Second, find someone with a diamond saw and cut it in half. Finally, take a hammer and a masonry chisel and score the rock all the way around. Then you can split it with the hammer and chisel.

Is this something you collected? If so, where did you find it? Also, if you can provide a photo we might be able to get some idea if it could be a geode just by looking at it.

Remember, all geodes are round rocks but not all round rocks are geodes.

[Pete Richards]

Another way to decide if you have a worthwhile geode or not is to evaluate its effective specific gravity. There are many websites that describe standard approaches to doing this. It is the weight of your geode in proportion to the weight of the same volume of water.

Quartz, the likely major component of a geode, has a s.g. of about 2.7. This is the measurement result you would expect if the geode is solid quartz (not what you want!). If the geode is hollow, its effective s.g. should be less because of the air space inside. You could even crudely estimate how hollow it is by comparing its s.g. with 2.7, since air effectively has no weight.

But you may not need to formally evaluate the s.g. If you find a "normal" cobble of the same size, preferably quartz or some other relatively light colored rock, you can compare its weight with that of your geode, either by hefting one and then the other until you can sense which is heavier, or by actually weighing both stones. A good hollow geode should weigh substantially less than a solid stone.

[davidcalhoun]

Thanks, I'll try that.

[Bob Harman]

Cracked, but unbroken geodes will, on occasion, bubble, but most will not. Truly unbroken examples will never bubble.
Geodes that rattle upon shaking are hollow; these were called Indian rattles or rattle rocks by Native Americans and given to their small kids to play with, hence the name.
Geodes that have a relatively large central cavity feel light in weight for their size while small cavity geodes are not easily told apart from solid ones.

To see 27 pages of pictures with maps and descriptions, mostly of Indiana geodes, go to the website: https://www.mindat.org and go to the posting thread of "MIDWEST SEDIMENTARY GEODES". You will have fun reviewing all the pictures and learn a lot from the descriptions. CHEERS......BOB

[davidcalhoun]

Bob- Thank you thank you thank you- that was a great link. I JUST went to Harrodsburg on 4/1/2016, I just missed you. I got some neat geodes, my best luck was at the small roadcut just south of the big huge one. David

[Reinhardt van Vuuren]

When selecting agates for lapidary slabbing we often use a technique of bouncing the stone in your hand and feeling the weight with which it lands in your hand, the general theory is the heavier the landing the more bands are usually present in the agate, in your case you would look for light weight but as others have mentioned this is only big cavities are easily detected this way. This technique does require some good practice but you will get the hang of it.

This technique is probably the most user friendly in the field and is rather rudimentary an sg test would obviously be much more accurate but this can be done at home on the ones you brought back.