What does Donne's "Undertaking" signify?!

Reading this poem I got confused about the details Donne wants to explain. I think he tries to represent a different kind of love, but what is it that should be hidden from others? Does he refer to the love without cosidering He or She. What does this signify? I have problem about the details and I cannot explain why he uses such metaphors or conceit. Can anybody help me interpreting this poem?

I HAVE done one braver thing
Than all the Worthies did ;
And yet a braver thence doth spring,
Which is, to keep that hid.

It were but madness now to impart
The skill of specular stone,
When he, which can have learn'd the art
To cut it, can find none.

So, if I now should utter this,
Others—because no more
Such stuff to work upon, there is—
Would love but as before.

But he who loveliness within
Hath found, all outward loathes,
For he who color loves, and skin,
Loves but their oldest clothes.

If, as I have, you also do
Virtue in woman see,
And dare love that, and say so too,
And forget the He and She ;

And if this love, though placèd so,
From profane men you hide,
Which will no faith on this bestow,
Or, if they do, deride ;

Then you have done a braver thing
Than all the Worthies did ;
And a braver thence will spring,
Which is, to keep that hid.

I HAVE done one braver thing
Than all the Worthies did ;
And yet a braver thence doth spring,
Which is, to keep that hid.

[The speaker here claims to have done a more courageous and more noteworthy thing than the famous Nine Worthies of antiquity.]

It were but madness now to impart
The skill of specular stone,
When he, which can have learn'd the art
To cut it, can find none.

[It would be pointless to show others how to work “specular stone” – a transparent material (selenite) used in ancient times because it was transparent and thus allowed light into buildings before the invention of glass windows – when you can’t find any of it around these days to work on.]

So, if I now should utter this,
Others—because no more
Such stuff to work upon, there is—
Would love but as before.

[So if he now discloses his secret – which as we see in stanza 5 is his selfless non-sexual or Platonic love for a certain woman – since she the woman he loves in this way is unique, and you won't find another like her, it would be the same as teaching the skills of “specular stone” – and since no one could find the raw material – another such woman - they would go on loving in the same old way]

But he who loveliness within
Hath found, all outward loathes,
For he who color loves, and skin,
Loves but their oldest clothes.

[But a lover who finds the treasure of a virtuous mind – “loveliness within” – forgets about the the physical “outward” aspects, which are superficial, like mere clothing. Because Donne's speakers vary in personality - and three of his poems are spoken by women - you will find other poems which entirely contradict this idea.]

If, as I have, you also do
Virtue in woman see,
And dare love that, and say so too,
And forget the He and She;

[the meaning is obvious here]

And if this love, though placèd so,
From profane men you hide,
Which will no faith on this bestow,
Or, if they do, deride;

[And if you hide your higher love for a woman from “lower” minds – “profane men” – who would never believe it any way, or if they did believe it, would criticize it - ]

Then you have done a braver thing
Than all the Worthies did ;
And a braver thence will spring,
Which is, to keep that hid.

[Obvious conclusion]

My take on it:

I HAVE done one braver thing
Than all the Worthies did ;

Kicks off with a surprising and potentially unappealing braggadocio - you dont know what he's done, and don't need to know who the 'Worthies' are to know he's boasting.

And yet a braver thence doth spring,
Which is, to keep that hid.

Ah - interesting... where's he going? He was boasting... but now he's saying Better to keep schtum about it... and yet, telling all that he should keep schtum... but you sense he appreciates the irony, so now you're in on the gag - and you've got a connection with his arched eyebrow and him tapping his nose.

It were but madness now to impart
The skill of specular stone,
When he, which can have learn'd the art
To cut it, can find none.

"Specular stone"... what's that? Scholars tell us it's selenite - who knows, who cares...? Its sort of the point that its something so arcane that few know about it. He's just picking some random lost art.

And maybe the master 'specular stone' cutter cant find any more 'specular stone' to cut. But it seems more poignant if the 'none' that he's referring to is not the 'specular stone', but the 'skill' - i.e. If you were a natural at 'cutting specular stone' (or any such thing which others find really difficult), but found no skill or joy in doing it, you might feel a bit dubious about saying how impressive you are for being able to do it...

So, if I now should utter this,
Others—because no more
Such stuff to work upon, there is—
Would love but as before.

"So if I make a big deal about my special achievement... its not going to win me any friends, is it? Far better to be modest... Having blown my trumpet once, best hush up before I lose the crowd - So I'll throw back in the red herring about the specular stone "stuff", which will hopefully throw people off the scent. Because if you're clever, you'll know its more clever not to let on how clever you think you are. No-one will "love" you for it (they'll just continue to love whatever they already loved).

But he who loveliness within
Hath found, all outward loathes,
For he who color loves, and skin,
Loves but their oldest clothes.

So now we're getting to the nub of it - the thing he's done, or 'achieved' that makes him believe he's so clever. First, he establishes his awareness that its 'fundamental' not 'superficial' qualities that matter...

If, as I have, you also do
Virtue in woman see,
And dare love that, and say so too,
And forget the He and She ;

...then establishes he's found a woman who possesses those qualities, which he's been capable of recognising, and that he loves her, and he's prepared to declare that love... and get so 'lost' in that love to the point where his and her identities merge... (as opposed to the 'loveless' skill of 'cutting specular stone' - which may be technically skilful, but is something you can either do or not - and anyway requires no enlightened strength of character to achieve)

And if this love, though placèd so,
From profane men you hide,
Which will no faith on this bestow,
Or, if they do, deride ;

...for all his euphoric urge to shout it from the rooftops (he knows he's said too much already - but he can't help himself), he also knows the most dignified and appropriate thing to do is to shut up about it. Since 'profane men' - who aren't worthy of such a woman, and don't 'get' what's important - would probably assume he's only interested in her looks, or her breeding, or her money, or some other triviality...

Then you have done a braver thing
Than all the Worthies did ;
And a braver thence will spring,
Which is, to keep that hid.

So if you're good enough for this woman, then you should also have the character not to forever drone on to everyone about what an achievement that is- it wont win you any friends, and they won't 'get' it anyway.