Here's what "A hard stone" could mean, depending on the situation:
* A stone with high hardness: Hardness is measured on the Mohs scale, with diamond being the hardest at 10. Some "hard stones" could be:
* Quartz: A common mineral with a hardness of 7.
* Granite: A rock composed of quartz, feldspar, and mica, with a hardness around 6-7.
* Diamond: The hardest natural material known.
* A stone that is difficult to break: This could be a dense stone like basalt, or a stone with a very tough structure like quartzite.
* A stone with a high compressive strength: This refers to the stone's ability to withstand pressure. Granite is a strong stone with high compressive strength.
* A stone that is difficult to work with: Some stones are very hard to cut or shape, making them difficult to work with for construction or jewelry making.
Please provide more context about what you're looking for! For example, are you interested in:
* Stones used in construction?
* Stones used for jewelry?
* Stones that are naturally occurring?
Once I understand what you're interested in, I can give you a more specific answer.