"Well, you'll get more gold" he replied, clearly slightly mystified at my stupidity.
"Yes, but what are the advantages of 18 carat gold over 9 carat" I said, clearly still appearing stupid.
"You get more gold" he said again. This conversation was clearly going nowhere, and he was getting irritated.
I gave up and went home.
I'm a practical person so owning gold for the sake of owning gold wasn't enough for me - I wanted to know a few more facts.
And after all, the difference in price between 9 carat gold, 18 carat gold and platinum engagement rings is pretty significant, so I didn't feel it was an unreasonable question.
For example, when I searched I found Amazon offering a 9 carat wedding band for £120 and a similar platinum ring for about £800. 18 carat gold generally costs twice as much as 9 carat gold rings.
You wouldn't buy a laptop or washing machine that cost six times as much as the rest without having some reassurance that it was 'better' than the cheaper alternatives.
So what's the answer, which metal is the best option?
The answer isn't as simple as you might hope, and there is some disagreement online about the various merits of 9 carat gold versus 18 carat gold.
According to H Samuel, 18 carat gold 'is much warmer and brighter in tone than 14ct and 9ct gold'. However, 9 carat gold is 'stronger and more durable and particularly suited to the creation of jewellery'.
However, you will find those online who say that 9 carat isn't actually more durable than 18 carat gold. For example, Australian jewellers Metal Urges says that 'The reason people believe 9ct gold to be more durable than 18ct gold is simple, there is confusion between the two terms 'hardness' and 'durability'.
There then follows a thorough explanation of the semantics of various apparent synonyms which will leave you scratching your head and giving up on shopping for another evening.
However, I think we can take heart from one single sentence within this article which sums up the futility of the whole debate: "An engagement ring made from 9ct gold can not be expected to last more than a lifetime's normal wear."
So, we can conclude that if you are happy for a ring to last for your wife's entire life then 9 carat gold will be fine. If you want it to last into the afterlife, then it might be better to pay another couple of hundred pounds for an 18 carat gold ring.
My view, as a practical person, rather than someone who loves to own expensive jewellery for the sake of it, is that 9 carat and 18 carat are both perfectly good enough for an engagement ring in terms of durability and quality.
If you are keen that your engagement ring has a 'brighter' tone then you might do better to choose 18 carat, but if the cost of marriage is putting you off proposing then I'd say 9 carat is a perfectly acceptable choice.
One other thing to consider - you should ideally choose the same type of gold ring for an engagement ring and a wedding ring. 18 carat gold rings are softer than 9 carat gold rings, so they will very very slowly rub against each other and wear away the softer ring.
Finally, let's consider platinum, where the advantages and disadvantages are rather less viciously debated.
H Samuel says that 'Many of us veer towards this metal because of its hardness and resistance to tarnishing'. An eBay article concludes that platinum 'never fades or changes colour' and I've not yet found many articles coming up with anything especially contradictory (I'm willing to be corrected on this).
You will though find that platinum is very expensive, and you may prefer to either let your gold ring lose a little colour over the years or get it replated at a jewellers (which will cost you somewhere in the region of £30, so far less than the extra cost of buying platinum).
If you're just starting out your search for an engagement ring, take a look at our guide to 10 things you need to know and our guide to 10 of the best places to buy cheap engagement rings under £1000, £500 and under £100.