Diamonds strong due to a property that we call “hardness”. While they are considered one of the hardest materials, the reason they can break is because they are not as strong when it comes down to “brittleness”.
To understand this in real world terms think of a plastic cup and a glass cup. The plastic cup can be easily scratched with a sharp metal object, but the glass will resist most scratching. This is because glass is more hard than plastic. Now lets think of what happens when you drop the glass cup and plastic cup from a reasonable height. The glass of course is going to shatter while the plastic will land safely in one piece. This is because glass is far more brittle than plastic.
Diamond isn’t as brittle as glass but due to having brittle properties it can still be shattered when hit right by a hard enough force. This is how they can use a chisel to “cut” or more accurately we should say they “chip” away sections of diamond.
For further information you can read more about hardness and softness by looking up “Moh’s Hardness scale” or if you want to learn about “brittleness” look that up or as opposed to being brittle you can read about a property called “ductile” which you may find interesting as well and that leads down a whole other rabbit hole. For advanced reading you may be interesting in learning about an engineering concept like Young’s modulus.
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