Help Identifying Rocks From Western Kentucky

[TexasNomad82]

When I was at Fort Knox’a training area I found this softball size rock. It weighs about 5lbs . No clue what it could be. Any direction would be appreciated.

[Geologyhound]

The bedrock in that area is primarily Mississippian age limestone formations with a capping sandstone member all overlaid with Quaternary alluvium.  Surficially, your stone has the appearance of a desert rose (selenite or barite). While there are apparently gypsum and anhydrite beds in the St. Louis Limestone which could be the source for selenite crystals, western Kentucky is not the right environment for the formation of desert roses.

However, the area around Muldraugh Dome is known for geodes.  Quartz geodes up to a third of a foot in diameter are common.  The Kentucky Geological Survey does say selenite and barite geodes are possible.

So, try to scratch it with your fingernail. If it scratches with your fingernail then it very likely is selenite.  If you can’t scratch it with your fingernail but you can scratch it with a copper penny (not zinc so use a penny before 1982) and it feels relatively heavy than it is probably barite.  If it won’t scratch with a copper penny, but the stone will scratch glass, then is a quartz geode.

[TexasNomad82]

Good analysis. Thank you. It does scratch with a pre-1982 penny and scratches glass. It’s rough so scratching with my fingernail is difficult but to make an observation. I’m torn between selenite and barite. It’s just a curious formation and no clue what it could be.

[TexasNomad82]

Thank you.

[Valens Legacy]

It also has a lot of quartz in it, which is common in that area.

[Geologyhound]

Sometimes it is difficult to discern the difference between scratching and breaking - especially when you have multiple minerals and/or grains/crystals held together in a semi-open framework or loose cementation.  But, with the capability of scratching glass, as Valens Legacy noted, you do have quartz.

Google Kentucky geodes.  Some of them can be quite pretty inside.  You can crack them with a hammer, but I prefer cutting with a rock saw.