Blue sapphire or Neelam gemstone can be worn, in between 3 to 6 carat as per experienced astroger advice. Clear and trasparent blue sapphire will give best result.
It can be worn in gold metal, if can not afford gold then can wear it in silver metal or panchadhatu metal ring. As it is associated with lord Shani, wear it on any Saturday of sukla paksha days. Early morning is the auspicios time, it should be worn in the middle finger right hand.
For purification and activating the gemstone, dip the ring in raw milk or not boiled milk, then in honey and pure water for 20 to 30mintutes. Burn insence sticks, kamphoor and offer any flower, with the name of SHani Dev to have the blessings of the lord. Then take out the ring from water and chant the Shani maha mantra "OMM SHANISHCHARAYE NAMAH" atleast 27 times. Then wear it in the middle finger of right hand.
Blue sapphire will start to give effect within 60 days after wearing. For good result you can wear a Ceylon blue sapphire. Defective and cheap gemstones may produce malefic results.
If you want to ensure if the gemstone will work good or bad, then just keeping it under pillow for sometime during sleep for the wearer. If he/she sees bad dreams then they must avoid to wear as it seems that the stone does not suits you. If no dream of get good dream during sleep, you can assume the gem stone is suitable atleast. Then as per Jataka/Kundali you can futher decide to wear it on the auspicious day.