As you like it, meaning behind quote?

Hi Peeps - this is my first post here and I registered specifically because I was curious about what the meaning is behind a Shakespeare quote from, As you like it.

Duke Senior:
Sweet are the uses of adversity,
Which, like the toad, ugly and venomous,
Wears yet a precious jewel in his head;
And this our life, exempt from public haunt,
Finds tongues in trees, books in the running brooks,
Sermons in stones, and good in every thing.

What I am wanting to know, is what is meant by the line, "Wears a precious jewel in its head"? What is the 'jewel' that is referred to?

I have my own idea of what it may mean, and I may be completely wrong, but before I share that I'd appreciate it if you peeps could share what you think it(jewel) means? It has been suggested to me already the 'jewel' might refer to the eyes of the toad which are jewel like, however, if that were the case I'd have thought that the bard would have said jewels rather than jewel.

Thank you in advance.