A diamond in the roof is a sentence best not to be used in the loving contexts of relationships as it may result in injury or diembowlement of one or both parties in the pairing.
To put into a working context, when declaring your feelings it is best not to describe said girlfriend as a diamond in the 'rough' for she will then look this up on the internet and beat the crap out of you. It is strongely advised therefore that the more appropriate expression "diamond in the roof" is used to avoid such ball bruisings :) Good luck fellow beans!
by Alexaura of the South January 15, 2012