Hard and tough, like the skin of an apple or the shell of a nut.
Scleroid things are very strong and not easy to break.
Examples:
The scleroid shell protected the nut inside.
The apple's scleroid skin was difficult to bite through.
The scleroid rock wouldn't break when I hit it.
The scleroid door was too strong to open!
The scleroid armor was so strong, nothing could get through it.
The scleroid tree trunk was as hard as steel.