Hi and welcome!
Unfortunately, stones usually don't go away on their own. None of the miracle cures has reliably worked out.
What you can do, however, is to change the diet to one lower in calcium. This is not a quick fix because it takes time for the body to work out any excess calcium.
Our diet guide looks at all aspects of a diet and also contains a chapter on recommendations for guinea pigs with urinary tract issues. Please be aware that often more calcium comes from water and pellets than from veg. It is important to look at the diet as a whole and not just at veg, which should only make about 10% of what a piggy eats in a day.
Here is the link:
Long Term Balanced General And Special Needs Guinea Pig Diets
There is also a very good chance that it is not a stone but a condition called sterile interstitial cystitis, i.e a non-bacterial recurring bladder infection which cannot be healed by antibiotics because it is not bacterial. The problem is that it can only be diagnosed be default after all other bladder issues have been excluded; including the possibility of a bladder stone or sludge.
It is not well known by vets that do not see lots of piggies but it has become rather common over the last 10 years and has overtaken UTI. It cannot be healed, only managed until it goes away on its own months or years down the line. The good news is that treatment is comparatively cheap and that it doesn't cause stones and is not fatal or life-shortening.
More information via this link here:
Guinea Lynx :: UTI
Management of IC is mainly with glucosamine, which we recommend for all longer term bladder problems, and metacam (ideally the stronger dog metacam rather than cat metacam). Glucosamine is not a medication but a food supplement. It is there to strengthen the beleaguered walls of the urinary tract system which are naturally coated with glucosamine as the urine is corrosive.
However, you will have to see the vet again first in order to exclude a stone/sludge before you can get a cavy sterile interstitial cystitis diagnosis. It is similar to feline sterile cystitis (FSC), which your vet may be more familiar with. What you cannot do and what doesn't work is treating for IC when you have a stone or sludge piggy. Once you know where you stand, you can then make an informed decision and treat accordingly.
PS: Glucosamine you can order from the internet (we recommend cystease which is a cat bladder food supplement; other brand names apply in other countries than the UK) or you can get it from a supermarket.
All the best!