The C has a hard sound in the word careful.
A fruit beginning with x is Xigua which is looks similar to a watermelon, only shorter in size. x Not sure about these: xerophyte - a plant that loses very little water and can grow in deserts; xeromorph - a plant that is adapted to salt-water marshes; and, xylocarp - a hard and woody fruit. x
Boulder ___ Could be GRANITE. That is a hard type of rock.
The word cinder starts with a soft consonant. When the 'c' sounds like a 'k' it is considered hard. When it sounds like an 's' it is considered soft.
The word "cycle" has a hard S sound (SY-kull).