The thing that gives it away is never the ring
People planning a surprise proposal custom ring worry about the wrong risk. They worry about the object being discovered. In practice the object spends its entire life in someone else's workshop and then a fortnight in a drawer.
What gives it away is behaviour. Being evasive about a Saturday. Asking her sister a question that has no innocent reason for being asked. Going quiet in a way you never go quiet. Six weeks of a custom commission is six weeks of small opportunities to be strange, and people notice strange long before they notice a receipt.
So the practical question is not how to hide a ring. It is how to keep the number of unexplained moments low, and a surprise proposal custom ring survives or fails on that rather than on concealment.
What actually has to stay secret
Less than you would think.
Ring size can be established without asking. A ring she already wears, borrowed for a day, is the reliable method — and it should be one from the correct hand, since fingers on the dominant hand are usually slightly larger. Bands can be adjusted afterwards within limits, and a ring that is slightly loose is a far better problem than one that does not go on at the moment it matters.
Taste is usually already documented. Look at what she wears now, and specifically at what she wears every single day rather than what she owns. If there is a shared shopping account or a saved folder somewhere, it is not snooping so much as research, and it will tell you more than any conversation would.
Timeline is the one people underestimate. A commission takes weeks, and it is better to start earlier and wait than to compress the making and spend the last fortnight anxious. Ask for the schedule in writing at the start so you can plan the date around the ring rather than the reverse.
The obstacle specific to us
Now the part that is our own problem to solve, and it is a real one.
A diamond grown from someone's carbon needs a sample of their hair. Roughly five grams, collected at a normal appointment. Which is fine, except that a surprise proposal means the person concerned cannot be asked, and taking a biological sample from an adult without telling them is not a charming piece of theatre. It is the sort of thing that sounds romantic in the planning and reads very differently when explained afterwards.
We will not do it. Not with a stylist's cooperation, not with a family member's, not for any reason. Consent is not a formality we would rather have and can proceed without.
Which is exactly what Pledge is for
So the line is drawn somewhere useful. Pledge is a commission grown from the giver's carbon only.
You provide the sample, from your own head, at your own haircut. Nobody else needs to know anything. The stone is grown, set, and handed over on the day, and the surprise survives entirely intact because the only person who had to be involved was you.
What it means as an object is arguably better than the alternative. A ring that carries some of your carbon is not a gift you selected. It is a gift that had to come from you specifically and could not have been bought by anyone else. She is not being handed a beautiful thing you chose. She is being handed a beautiful thing that is partly you, made in the weeks you spent deciding to ask.
It carries no DNA, and the sample takes part in the growth rather than making up the whole of the stone — we say both plainly on every page rather than let anyone find out later.
And afterwards, if you want the other thing
Plenty of couples want a ring that contains both of them. That form is the two-stone Union ring, and it requires a sample from each partner, which by definition cannot be a surprise.
The usual solution is sequential rather than either-or. Propose with the Pledge ring. Then, once the answer is yes and the secrecy has served its purpose, commission the pair of stones together as wedding rings or as a later piece — the two-stone entry covers why that form makes the strongest case for it.
You end up with two objects rather than one, marking two different moments: the one where you decided alone, and the one where you decided together. That is not a consolation prize.
Small practical matters
Have it delivered somewhere other than home, or have it held until the day. Insured and signed for means somebody has to be in, and the person who is in is very often the person who should not be.
Tell one person, not four. A second opinion on ring size is worth having. A group chat is a leak with extra steps.
And decide in advance what you will say if you are asked directly. Not a lie exactly — an answer that is true, boring, and closes the subject. People who have not thought about this in advance improvise, and improvisation is where it goes wrong.
Where to start
If the date is fixed, start with the date and work backwards. Tell us when, and we will tell you honestly whether it is achievable rather than agree and hope. If it is not, we would rather say so while you still have time to do something else.