Not planning to retire on the proceeds, just interested and couldn't
find details online, that's all ;-)
It is well made, definitely not cheap and nasty and the hallmarks are
very nice which is why I thought that it could be real gold.
She paid £2 for it and is really happy to wear it, regardless of its value.
Based on what was suggested here, it is very likely a Swarovski. Not
available on their website for sale, but the cheapest ring there (and
only very few of them are real gold) is around £60, some a LOT more, so
a bargain nonetheless.
Thanks all for you help!