Actual content of the quote
First, the original quote in the Cambridge Edition, as it is a lot more truthful to the original than your quote, which reads like a paraphrase:
Even if through the peculiar disfavor of fate, or through the
meager endowment of a stepmotherly nature, this will were entirely lacking
in the resources to carry out its aim, if with its greatest effort nothing of it
were accomplished, and only the good will were left over (to be sure, not a
mere wish, but as the summoning up of all the means insofar as they are in
our control): then it would shine like a jewel for itself, as something that has
its full worth in itself. Utility or fruitlessness can neither add to nor subtract
anything from this worth. It would be only the setting, as it were, to make it
easier to handle in common traffic, or to draw the attention of those who are
still not sufficiently connoisseurs, but not to recommend it to connoisseurs
and determine its worth. (Groundwork for the Metaphysics of Morals, Ak. 4:394, Cambridge p. 10)
It is all about the value of a good will, which is absolute, whereas usefulness is always relative and depending on actually willing the end. This is explicitly addressed later:
All objects of inclinations have only a conditioned
worth; for if the inclinations and the needs grounded on them did not exist,
then their object would be without worth. The inclinations themselves, however, as sources of needs, are so little of absolute worth, to be wished for
in themselves, that rather to be entirely free of them must be the universal
wish of every rational being. Thus the worth of all objects to be acquired
through our action is always conditioned. (Groundwork for the Metaphysics of Morals, Ak. 4:428, Cambridge pp. 45-46)
Regarding the metaphor
I think the reason why Kant is using this picture here is that Jewels are, or at least have been for commoners, the closest to what he calls absolute worth. The setting, e.g. in a ring, crown or necklace, does not really increase its worth, it just makes it more handy. People knowing the worth of Jewels know this and only look at the Jewel itself in order to determine the worth, but others will only consider the worth of the piece of jewelry as a whole, as they would not be able to admire the jewel as such as it would be quite useless in their eyes without the setting.
That's how it works with the good will as well: People understanding morality admire the morality and goodness of the will, while others always ask the question what positive outcomes he could expect, i.e. how useful his actions were.
The main point to see here that Kant wants to point out in this part of the book is that while morality does not exclude usefulness (the jewel may have a setting), the worth of an act is only determined by the good will (the jewel that shines for itself) and usefulness must not be considered for evaluating the moral worth of an act (the setting must be set aside). But for people always asking for usefulness, the setting may be useful (sic!) in order to at least acquaint with the shining of the jewel/morality (and the admiration/respect it produces).