I would try white vinegar, soak them for a while in the vinegar and then rinse thoroughly in hot water and then scrub them again with a soapy solution. It is a good idea to put the stones outside in the sun occasionally to recharge them.

Rancid stones is an impossibility...stones don't spoil and have no known bacteria to create odor soooo...it is rancid oil from improper cleaning and using the wrong oil...(if the stones arrive from supplier with odd smell it is grinding/polishing grout and can be removed by tossing them in the dishwasher..best to put them in a wire mesh basket or strainer with very small ones in the silverware tray....)
The only Geothermal Therapy Association approved oil for stones is actually a plant extract...Jojoba you can find it on Google...reason?...anytime you add warm temperatures you dilate pores and the product will be delivered much quicker into the entire circulatory system. If you know your contraindications well and have done a complete intake, you may use other products to serve special needs for the client...planning of course to properly sanitize the stones after use...if you are not sure about all the properties in your oil or lotion DO NOT USE IT WITH WARM TEMPS...it will enhance all active ingredients and you must know what the physiological action will be ...aside from that Jojoba oil (plant extract) is not an oil, will not go rancid, never spoils, does not smell up the linens, does not stain fabric, washes off stones and all equipment instantly and is the closest thing to the skins natural oils....
Some oils and lotions will actually bake onto the stones like varnish....once this happens you must soak them in a paint remover or solvent (acetone) to soften the varnish enough to remove it from the surface...this should be done out doors ...soak them for 24-48 hours...remove stones and scrub with a brush in hot soapy water, rinse in clear water and dry to be sure all sticky film is removed and smell is gone...then they should rest on the ground over night to recharge their batteries...(think I'm kidding?...think magnetic field, balance of alkaline/acid surface and electromagnetic energy....if the stones have been through all this, they will be neutral and will not hold heat.....this is a horrible thing to have to do and it makes you gag to be around that much chemical. But this is the only way we got some clean that were being used in a spa in DC in 2004 before folks learned not to use olive oil!!
Good luck...

Thank you Jenny for sharing this most interesting information.

Jenny Ray said:

Rancid stones is an impossibility...stones don't spoil and have no known bacteria to create odor soooo...it is rancid oil from improper cleaning and using the wrong oil...(if the stones arrive from supplier with odd smell it is grinding/polishing grout and can be removed by tossing them in the dishwasher..best to put them in a wire mesh basket or strainer with very small ones in the silverware tray....)
The only Geothermal Therapy Association approved oil for stones is actually a plant extract...Jojoba you can find it on Google...reason?...anytime you add warm temperatures you dilate pores and the product will be delivered much quicker into the entire circulatory system. If you know your contraindications well and have done a complete intake, you may use other products to serve special needs for the client...planning of course to properly sanitize the stones after use...if you are not sure about all the properties in your oil or lotion DO NOT USE IT WITH WARM TEMPS...it will enhance all active ingredients and you must know what the physiological action will be ...aside from that Jojoba oil (plant extract) is not an oil, will not go rancid, never spoils, does not smell up the linens, does not stain fabric, washes off stones and all equipment instantly and is the closest thing to the skins natural oils....
Some oils and lotions will actually bake onto the stones like varnish....once this happens you must soak them in a paint remover or solvent (acetone) to soften the varnish enough to remove it from the surface...this should be done out doors ...soak them for 24-48 hours...remove stones and scrub with a brush in hot soapy water, rinse in clear water and dry to be sure all sticky film is removed and smell is gone...then they should rest on the ground over night to recharge their batteries...(think I'm kidding?...think magnetic field, balance of alkaline/acid surface and electromagnetic energy....if the stones have been through all this, they will be neutral and will not hold heat.....this is a horrible thing to have to do and it makes you gag to be around that much chemical. But this is the only way we got some clean that were being used in a spa in DC in 2004 before folks learned not to use olive oil!!
Good luck...