This isn't really a place for relationship advice, but to frame it in the context of a startup, if a founder just won't talk about a decision you both need to make together, that's a huge red flag that maybe going into business with this person isn't the best idea.
I haven't gone through this myself, but I've read that part of picking a great founder is finding someone you can talk through issues with, even if they're uncomfortable.
Not talking about something is a much bigger issue than disagreeing about something.
Take it from another perspective, this is more like a cofounder (or really, two cofounders) being so headstrong that he won't budge on his opinion for the good of the company.
If you explain clearly your viewpoints on diamonds, where those viewpoints are rooted, and the partner doesn't agree even after understanding your viewpoint, then you should consider compromising.
I haven't gone through this myself, but I've read that part of picking a great founder is finding someone you can talk through issues with, even if they're uncomfortable.
Not talking about something is a much bigger issue than disagreeing about something.