(The photo below is a rare one of me with reasonable looking nails....they are usually ragged and scruffy from being in the workshop.....one of the only downsides to silver smithing!)
They didn't have any marquise cabochons for sale but the elderly chap behind the counter produced a tray of stones amongst which were some oval 20 x 10mm cabochons, labradorites, carnelian, aventurine, and jade. They were only around £1 each, (I would normally expect to pay £3 minimum, even as much as £6 for similar stones elsewhere) so I bought most of them.
For the carnelian ring I used some sterling silver D wire for the shank, and cut out the back of the bezel base to let the light shine throught the stone. Just sold my third one of these by word of mouth which is great!