I looked it up. Here's what I found.
Diamonds have been a source of fascination for centuries. They are the hardest, the most imperishable, and the brilliant of all precious stones. The word "diamond" comes from the Greek word adamas, meaning "unconquerable".
I know many of the people who are the most prophetic, many of whom are diamonds, who have been thru the fire, who have been hardened, who are unconquerable. There are a couple other features of diamonds that fit this as well. They are hidden. They look like every other rock. When split and polished they begin to shine, reflect and become what they always were....Diamonds.
The church of Jesus is somewhat like this. Not everything the Church does is diamond-like. I am like Spurgeon, I hate the idea that some people don't take the whole gospel at face value and instead pick and choose doctrines they like and hold them up as diamonds ignoring the covered true diamond truths buried in the earth. They aren't diamonds. I think you have to let the whole text tell the story. Spurgeon had it right.
Be well instructed in theology, and do not regard the sneers of those who rail at it because they are ignorant of it. Many preachers are not theologians, and hence the mistakes which they make. It cannot do any hurt to the most lively evangelist to be also a sound theologian, and it may often be the means of saving him from gross blunders. Nowadays we hear men tear a single sentence of Scripture from its connection, and cry 'Eureka! Eureka!' as if they had found a new truth; and yet they have not discovered a diamond but a piece of broken glass." - Charles Spurgeon