noun, の adjective
A noun (名詞) that has hardness is a noun that can't be budged. It's a big noun, so big that you have to capitalize these kinds of nouns in English. It is a proper noun.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. It will help to know the word 名詞 as well.
You can’t use proper nouns in the game Shiritori.
How are proper nouns different from nouns?
I eagerly looked up a word that I didn't recognize in the dictionary, but couldn't find it and it turned out it was just a proper noun.