Okay, there are three forms:
A's B (ex. the car's engine)
B A (ex. a car engine)
B of A (ex. the engine of the car)
The first form indicates "possession". B belongs to A.
Perhaps I need to clarify that the word "possession" here is used very loosely. It could refer to almost any sort of relationship. If I say, "Fred's pencil", I mean the pencil that Fred owns, that belong to him and that he can do pretty much anything he wants with. If I say, "Fred's city", I probably do not mean that Fred owns the city and can do whatever he wants with it. I probably mean something more like, the city where he lives.
The "B A" form indicates a "type". When I say, "a car part", I mean a part that is of type "car". In this case, a part that comes from a car or that is normally used in a car. The difference between this and "B's A" can be subtle, but in general, the "B's A" indicates association with a particular thing, while "B A" indicates an association with that type of thing in general. Like if I say, "This is my car's battery", I mean that it is the battery for this particular car. But, "This is a car battery" means it is a battery intended to be used in a car, but which I am not (presently) associating with any particular car.
So, "the Iron Age" means an age that is associated with iron. Not a particular piece of iron, just iron in general.
"A of B" can be used in place of either of the previous two forms. Sometimes we distinguish by the use of articles, pronouns, or other adjectives. Like, "this is the battery of my car" is the same as "this is my car's battery". Other times we just tell by general context. "This was the Age of Iron" is the same as "this was the Iron Age", but "This was the Era of Napoleon" is the same as "this was Napoleon's Era".
Whether you use the "of" form or one of the other two forms just depends on what makes the sentence flow smoothly. Sometimes a sentence is more clear with one form than the other, or just has a nicer rhythm.
Best Answer
This construction X is of Y can be useful if you want to concisely qualify the interest, value, etc. with an intensifier.
Example:
which equals
“of little interest” equates to “is not very interesting”. It does not equate to “is not interesting”, nor to “is uninteresting”: that would be expressed as “is not of interest”.
“of very little interest” roughly equates to “is very uninteresting indeed”, or “is extremely uninteresting”.
You can, however, go one further and say “…of very little interest indeed”, which means something like “…is not interesting in the slightest”.
It would be unusual to use small here: …of small interest. That feels correct but very literary. Conversely, you can say “…of big interest” in speech, but perhaps not in writing: it feels clumsy. You can say …of huge | massive interest (in speech), you would be unlikely to say …of tiny interest (avoid).
You may have noticed that it's probably easier to figure out these variants:
and these negations
than it is to figure out these equivalents
and
The pattern I have shown for of interest and its variants can be directly applied to
However, to work out the adjectival forms of those attributes or characteristics is sometimes trickier and less regular: for example, there is no commonly used word unvaluable (avoid). Although it is in the dictionary I have never heard or seen it used: you would normally say
(Note that invaluable means "so valuable that you cannot count its worth", "extremely valuable".)
So the good news is that you can always use a very regular pattern for of Y and its qualified variants and their negations.
The bad news is you may sound a bit like you're talking like a book, or like someone from the 1950s. But you will sound like you are well-educated :-).
My final comment would be: you can't use this construction with any noun that could be considered to be an attribute of the thing X. This goes to the earlier observation that Y must be an abstract noun