yes, one method is based no palmistry as you said about mount of saturn, mount of mercury etc..
the result of that is as said by riaz ji...
based on friendship also could be worn as said by rishi ji.
for example--mount of venus is located at the bottom of Thumb, but generally no one wants to wear a ring on the thumb. Saturn is a friend of Venus. so its worn on middle finger which is of Saturn's mount.
Traditional astrological method prescribes another method..
The purpose for which one wants to wear the Gem i.e.
Dharma, Artha, Kama, Moksha.
For Dharma- Ring finger
Artha - Middle Finger
Kama - Little Finger
Moksha - Index Finger
There might be other methods too....