The expression "is of" means 'has the characteristic of' or simply 'has'. 'of + noun' can be used to say that someone or something has a particular quality. "is of" is a form of "to be" followed by a noun or an adjective and a noun. be + of + adjective + noun (abstract) ---> e.g., to be of good cheer = to be cheerful The noun is usually an abstract noun. ************************************************************************************************** Abstract nouns are defined as: "A noun that refers to states, events, concepts, feelings, qualities, etc., that have no physical existence. They might seem real to our senses of sight, smell, hearing, taste, or feel, but “they cannot be held in your hand.” For example: State: cleanliness / impairment / emergency / freedom / excitement / calm Events: birth / death / anniversary / graduation / celebration / meeting / competition Concepts/Ideas: liberty / freedom / democracy / communism / opportunity / sound / music Feelings: happiness / sadness / joy / love / hate / anger Qualities: beauty / honesty / kindness / integrity It is not possible to measure or count abstract nouns…how much love, how much freedom, how much honesty (not “how many love, how many freedom, how many honesty – these abstract nouns are uncountable). ************************************************************************************************* "is of ..." to be of use = to be useful to be of help = to be helpful to be of importance = to be important to be of significance = to be significant to be of consequence = to be consequential to be of interest = to be interesting to be of assistance = to be helpful (because there is no word "assistanceful") to be of worth = to be valuable (because there is no word "worthful") to be of good quality = to be good with respect to quality The ore discovered by the expedition is of a high quality = The quality of the ore, discovered by the expedition, is high.

You may use "is of" when you're describing something. For example in your sentence, which by the way is entirely correct, you're describing the ore by saying it has high quality. Similarly, you could say "this person "is of" latin american background", once again describing the subject in the sentence. Now, if you encounter "is off' with a double f, the meaning changes and it means "coming from". Like in the sentence "That line is off the movie 'The Titanic' ".

It is correct. Another way of saying is : The ore .....is of high quality. The ore ..... is high-quality.

Correct. It sounds a little academic/formal, probably because it's written in the passive voice, and because you're assuming the reader knows what type of ore you're talking about, but it's 100% ok.

The sentence is fine. "Is of" is a bit of an expression. These mean essentially the same thing: The ore is of a high quality. The ore is high quality. The only distinction might be is that the first emphasizes that the ore is a member of a group of high quality ores, not just that it itself is high quality. But this is splitting hairs. Think of them as the same.