A ring in dreams and visions symbolizes a person’s commitment to another person. It speaks of unity or an agreement or a covenant.
A gold ring symbolizes a person who is committed to walking faithfully.
Diamonds in dreams symbolize spiritual wisdom but also hardness and spiritual riches and brilliance. Wisdom speaks of the wisdom of God and his word and hardness speak of a person being spiritually strong which is also symbolized by a hard forehead.
The color black in dreams and visions symbolizes unbelief or depression or fear or evil or separation from God. Black also symbolizes evil or demonic activities.
The signet part (initials as a seal) speaks of authority to ratify something like a document. In your dream, it could be that you, as the wearer of the dream have some legal authority in the spiritual realm, pertaining to your father.
The hands of a person symbolize the power and organizational aspects of the body of Christ. It is all about power (opportunity and ability to bring into existence) organizing, functioning, and the handling of spiritual issues in the body of Christ. It symbolizes strength, power, possession (in one’s hand), and action. In this case, this is pertaining to your father.
The thumb is the only finger that can easily touch all the other fingers. This has a spiritual connotation. It speaks of ability power and authority (in this case, I am not referring to the function of an apostle).
Before you make any conclusions, in the spiritual realm, we are connected to God the Father and not to our earthly fathers. So, this dream is not putting any obligation on you of any sort. This dream is a representation of your loyalty and commitment to your earthly father. It is a good thing and commendable. The fact that he is not saved did not change anything. To you, he is a black diamond. That speaks of something precious even though he (your father) is not saved (black). Your commitment and loyalty toward him are still faithful (gold) and you will not forsake him. You will do anything within your power to get him saved (thumb).
The fact that it is your left thumb and not the right speaks of you feeling limited pertaining to what you can do about the situation. You are influenced negatively and that influences the actions that you can take. If it was the right thumb, the emphasis would have been on your ability to act the right way and carry out whatever needs to be put in place with authority and power.
The fact that the diamond is black also speaks of the wrong wisdom of your father. He does not have the true Godly wisdom but false wisdom.
So the situation is hard for you but yet you decided to be loyal and to do whatever you can to change your father’s life for the better (to get him saved).
Keep on praying for him and even fast if you can. Keep on asking God to forgive his sins, even confess his sins on his behalf (nothing to do with generational curses)(Just say, God, please forgive his sins, and then you mention the bad things that you know of). If you persist, maybe the negativity can be turned around and you can make breakthroughs in this regard. Never forget, the responsibility for his salvation is his. Not yours. You can pray for him and intercede for him but the responsibility and accountability remain his.