Strong and hard materials, like rocks and stones, are difficult to scratch or break.
There are plenty of words we throw around in everyday speech that take on different, much more specific meanings when they're used in a technical context. One place you'll find a bunch of them is in the realm of material science.
This fantastic primer video from Real Engineering highlights a few in particular: stiff, tough, strong, ductile, brittle, and hard. It's a great rundown for newbs, or refresher for anyone who hasn't seen Young's modulus in a while.
The video provides a lot of valuable context, but the basic vocab list breaks down roughly like this:
Stiff — resists deformation
Tough — resists failure, even after deforming
Strong — resists both deformation and failure
Ductile — deforms before it breaks
Brittle — breaks before it deforms
Hard — resists dents, scratches, and other permanent changes under compressive force
These qualities aren't necessarily good or bad because different problems need different solutions. What good is a spring that doesn't have some give to it? So the next time you use one of these words, just take a second to stop and think about what it actually means.