El marco es de oro.

Because Spanish usually uses a definite article where English often uses the.

• el marco = the frame
• Saying just marco would sound less complete here unless you were speaking in a very specific context, like labels or notes.

Also, marco is a masculine singular noun, so it takes el.

You usually learn the noun together with its article:

• el marco
• not la marco

There is no perfect rule for noun gender, but many nouns ending in -o are masculine. Here, the correct form is el marco.

Because this sentence describes an essential characteristic: what the frame is made of.

• ser is used for identity, classification, origin, material, and inherent characteristics.
• estar is used more for location, condition, or temporary states.

So:

• El marco es de oro = it is made of gold / it is gold
• not está de oro

Here de means made of or of in the sense of material.

So de oro tells you the material of the frame.

This is very common in Spanish:

• de madera = made of wood
• de plástico = made of plastic
• de metal = made of metal

Yes. El marco está hecho de oro is also correct and often more explicit.

Compare:

• El marco es de oro = the frame is of gold / made of gold
• El marco está hecho de oro = the frame is made of gold

The version with está hecho de emphasizes the idea of construction or composition a little more, but both are natural.

Usually, yes: it suggests the material is actually gold.

If you want to say gold-colored rather than made of real gold, Spanish often uses:

• dorado = golden / gold-colored

So there is an important difference:

• de oro = made of gold
• dorado = gold-colored

Because after de when talking about material, Spanish normally uses the noun without an article.

• de oro
• de madera
• de plata

Using del oro would usually sound wrong here.

No. oro is a noun here.

In de oro, the noun oro is part of a prepositional phrase that describes the material.

So grammatically:

• El marco = subject
• es = verb
• de oro = complement telling what material it is

The normal order is:

• El marco es de oro.

You could change the order in poetry, emphasis, or very marked speech, but for normal conversation and writing, this is the standard and best order for learners.

It is pronounced roughly like OH-roh.

A few details:

• the r in oro is a single tapped r, not a strong rolled rr
• both o vowels are clear and pure, not reduced like English vowels sometimes are

So:

• o-ro
• with stress on the first syllable: O-ro

Yes. It follows a very useful pattern:

[definite article] + [noun] + ser + de + [material]

Examples:

• La mesa es de madera.
• El anillo es de plata.
• La botella es de vidrio.

So El marco es de oro is a model sentence you can copy with many other materials.

Yes. Marco can have other meanings depending on context, such as framework or setting, and it can even be a male name, Marco.

But in this sentence, with El marco es de oro, the most natural meaning is the physical frame.

Yes, it is grammatical and natural.

That said, in everyday conversation, speakers might also choose:

• El marco es de oro.
• El marco está hecho de oro.

Both are fine. The first is simpler and very common as a grammar pattern; the second can sound a little more explicit in everyday speech.