There are different options for lubricants to use for Ruby Cup and also Ruby Kegel.
Medical grade silicone does not get affected/altered in any way by silicone lubes given that they are lubes made of 100% silicone with no additives etc.
Any water based lubricants with natural ingredients are OK to use too.
As a more natural option you can use a little bit of Organic Virgin Coconut Oil as a lubricant.
The most natural option however would be to use just a little bit of saliva, though it might not be everyones favourite option :)