Who or what is this poem referring to?

Don't confuse me with a line
A line may contain me
And I may contain a line
But that's not my identity

I have one best friend
Though I don't often appear
When I do show up
He's almost always near

I can be round and tall
And may touch facial hair
But even when I'm alone
You may still see two there

Find me in ancient places
I am worth quite a lot
I can bring about the end
But I'm rarely in that spot

Who or what is this poem referring to?

Don't confuse me with a line
A line may contain me
And I may contain a line
But that's not my identity

I have one best friend
Though I don't often appear
When I do show up
He's almost always near

I can be round and tall
And may touch facial hair
But even when I'm alone
You may still see two there

Find me in ancient places
I am worth quite a lot
I can bring about the end
But I'm rarely in that spot