Iolite mainly suggested for Makar(Capricorn) and Kumbh(Aquarius) Rashis in Vedic astrology and Virgo in the Western Zodiac. 

Weight:     Typically, the gemstone size should be approximately 1/10th of your body weight, that means 1 carat for every 10 kgs. Alternatively, it can be according to the strength required as per your horoscope or as advised by your astrologer.
Quality:    For the best astrological results, unheated and untreated iolites found in India and Madagascar are ideal.  
Metal:     Silver or pital (brass)
Finger:     The middle finger of the left hand for women and right hand for men is our recommendation. 
Day:        Saturday
Time:        Evening after sunset at 7.15 PM/ 8.15 PM/9.15 PM as Saturn is most potent after sunset. 
Mantra:    Om Shani Namah or Om Shanicharya Namah
 

Benefits                          
It is the preferred upratna to Blue Sapphire as it benefits the wearer with a rise in status, health recovery, and an increase in fame and popularity.  

Career – It awards the hardworking with a steady rise in their professional conquests. 
Reputation – Success begets popularity, and an Iolite propels an excellent social standing. 
Addiction – It helps overcome negative thoughts and addictions with its healing energies. 
Health–It enhances the sleep patterns leading to better mental and physical health. It also is known to cure sore throat, varicose veins, and skin blisters. 
Consciousness – It helps balance the energies and strengthens your aura with heightened perception powers. 

Iolites are mined in Australia, Brazil, Burma, Canada, India, Madagascar, Sri Lanka, Namibia, Tanzania and the USA. Among these, Indian and Madagascar Iolites score over others.Light to deep blue gemstones with a purplish tinge is considered ideal. They need to retain their pleochroism phenomenon for astrological power. Hence the cut, in this case, matters. Slight mineral inclusions and metallic platelets are a given. Rare iolites with brown to reddish platelets are known as bloodshot iolites.