Okay, the numbers and units got a bit mixed up here, I think. The karat (BE 'carat', not carrot, like the vegetable) measures fineness, not mass, with 24 karat denominating gold with a purity of 999 parts per thousand (i. e. millesimal fineness).
Now, there is also another form of carat (note the difference in spelling), the so called metric carat, equal to 200 mg (0.00705 oz) or 0.00643 troy oz. This one measures weight, but is used exclusively for measuring gemstones and pearls. But of course, technically one can measure everything with such a unit, be it gold, sugar or grain.
The first unit of karat cannot meaningful give an answer to the question of "how much do 40 karats of gold weigh", because a) karat do not measure weight and b) karat only go up to 24, everything purer than 24 karat gold will be exclusively measured in parts per thousand. The finest gold ever refined was 999.999 parts per thousand pure, but regular production doesn't go beyond 999.99 parts per thousand (looking at you, Royal Canadian Mint).
The second unit of carat could technically measure gold, but isn't used that way. However, 40 carat equal 8 g, which equal 0.17637 pounds or 0.282192 oz. That's a good weight for a ring and would give enough width and thickness to set a stone.
However, should the sapphire itself have 40 ct, then it would not fit a ring, in my opinion, at least not aesthetically pleasing. If I calculated correctly a 40 ct sapphire in cubic shape should have an edge length of more than 3.5 cm (ca. 1.38 inches). The diameter would grow, of course, if cut into a flatter shape, e. g. round, oval or marquise.
Prices are different to calculate for Traveller, though, since demand and supply are not known. But 8 g 999 gold (or 24 karat) today sell for less than 400 USD. Add the price to make a ring out of it, give the gold smith and the dealer their cut and you might come out at a market value of 1,000 to 2,000 USD today. Probably closer to 1,000 USD, though, since gold rings are mass products. I paid around 800 € for my ring of 750 or 18 karat gold, and I guess it weighs around 5-6 gram.
The sapphire on the other hand would be almost invaluable at 40 ct. Stones like this aren't usually sold on the market, but cut down into smaller gems. Only very rare finds are crafted into single stones, e. g. as center pieces of royal jewlery. If offered for sale, prices can be astronomically. The Hope Diamond for example has 45.52 ct and sold for 185,000 USD in 1911. Adjusted for inflation, that's more than 5 million USD today.
Putting such a stone on top of a mass produced 1,000 dollar ring would be a total shame, and actually devalue the gem. So, maybe you want the ring to be handcrafted by a renowned artisan. That coul up the price considerably, easily adding a zero. It'll still be a monster stone on that ring, though.
Since Imperial Credits aren't USD 1:1, but probably closer to 1:5 (if 1 Cr was supposed to be 1 USD in 1977), the whole ensemble could be worth 1 MCr or more, depending on its place in history and the volatility on the market.