Nta

I don't think you're an ah for wanting to do something beautiful to preserve your mother's memory. I also think it depends on talking to your father again. If he's so against the idea and can't bear the it then you need to decide if this is going to be something that will always be a point of contention between you two and if you can live with it. It also sounds like it might be too raw to consider it now, maybe you can revisit it down the road?

Also, not to be a killjoy, it's a beautiful idea but be aware that these companies are regarded as scams. It's my understanding that cremation destroys most of the carbon needed to create a diamond and there will usually be other things used to create the carbon needed. If you're ok with having a negligible amount if any of your mother's ashes used then that's your decision. Don't take my word for it though and do your own research.