You're a

Rainbow

Don't confuse me with a line

Don't worry, you're a "bow"

A line may contain me

Light rays can be seen as a straight line, but if pointed on a prism, a rainbow is revealed

And I may contain a line

E.g. the peace flag is composed by lines of different colors.

But that's not my identity

The most famous rainbow is the one created by rain.

I have one best friend

Sun

Though I don't often appear

When it rains, not always a rainbow appears

When I do show up
He's almost always near

Only when there is sun and rain a rainbow can occur

I can be round and tall

Round and in the sky

And may touch facial hair

YEAH! (Really no clue here...)

But even when I'm alone
You may still see two there

Double rainbow!! (actually is only one)

Find me in ancient places

There are a lot of references to rainbows in ancient mythology

I am worth quite a lot

Mmmh, I think this one is a hoax.

I can bring about the end
But I'm rarely in that spot

Have you ever tried to chase the rainbow? Even if an observer sees another observer who seems "under" or "at the end of" a rainbow, the second observer will see a different rainbow—farther off—at the same angle as seen by the first observer.

You're a

Rainbow

Don't confuse me with a line

Don't worry, you're a "bow"

A line may contain me

Light rays travel on a straight line, but if pointed on a prism, a rainbow is revealed

And I may contain a line

Rainbow colors are always represented as straight lines. E.g. the peace flag is composed by lines of different colors. Our parents (and sadly also me) know it best.

But that's not my identity

The most famous rainbow is the one created by rain.

I have one best friend

Sun

Though I don't often appear

When it rains, not always a rainbow appears

When I do show up
He's almost always near

Only when there is sun and rain a rainbow can occur

I can be round and tall

Round and in the sky

And may touch facial hair

YEAH! (Really no clue here...)

But even when I'm alone
You may still see two there

Double rainbow!! (actually is only one)

Find me in ancient places

There are a lot of references to rainbows in ancient mythology

I am worth quite a lot

Mmmh, I think this one is a hoax.

I can bring about the end
But I'm rarely in that spot

Have you ever tried to chase the rainbow? Even if an observer sees another observer who seems "under" or "at the end of" a rainbow, the second observer will see a different rainbow—farther off—at the same angle as seen by the first observer.

You're a

Rainbow

Don't confuse me with a line

Don't worry, you're a "bow"

A line may contain me

Light rays can be seen as a straight line, but if pointed on a prism, a rainbow is revealed

And I may contain a line

E.g. the peace flag is composed by lines of different colors.

But that's not my identity

The most famous rainbow is the one created by rain.

I have one best friend

Sun

Though I don't often appear

When it rains, not always a rainbow appears

When I do show up He's almost always near

Only when there is sun and rain a rainbow can occur

I can be round and tall

Round and in the sky

And may touch facial hair

YEAH! (Really no clue here...)

But even when I'm alone You may still see two there

Double rainbow!! (actually is only one)

Find me in ancient places

There are a lot of references to rainbows in ancient mythology

I am worth quite a lot

Mmmh, I think this one is a hoax.

I can bring about the end But I'm rarely in that spot

Have you ever tried to chase the rainbow? Even if an observer sees another observer who seems "under" or "at the end of" a rainbow, the second observer will see a different rainbow—farther off—at the same angle as seen by the first observer.

You're a

Rainbow

Don't confuse me with a line

Don't worry, you're a "bow"

A line may contain me

Light rays can be seen as a straight line, but if pointed on a prism, a rainbow is revealed

And I may contain a line

E.g. the peace flag is composed by lines of different colors.

But that's not my identity

The most famous rainbow is the one created by rain.

I have one best friend

Sun

Though I don't often appear

When it rains, not always a rainbow appears

When I do show up 
He's almost always near

Only when there is sun and rain a rainbow can occur

I can be round and tall

Round and in the sky

And may touch facial hair

YEAH! (Really no clue here...)

But even when I'm alone 
You may still see two there

Double rainbow!! (actually is only one)

Find me in ancient places

There are a lot of references to rainbows in ancient mythology

I am worth quite a lot

Mmmh, I think this one is a hoax.

I can bring about the end 
But I'm rarely in that spot

Have you ever tried to chase the rainbow? Even if an observer sees another observer who seems "under" or "at the end of" a rainbow, the second observer will see a different rainbow—farther off—at the same angle as seen by the first observer.