You can select your gem according to your star or Zodiac sign. The Zodiac or a circle is of 360 degree. Each division or a house or Rasi (as known in India) is of 30 degrees. There are 12 divisions of the Zodiac and each has a different ruling planet. The 12 solar months are named after the 12 Zodiac signs. Zodiac is divided into 12 equal divisions of 30 degrees. The 30 degrees is again divided for 2 2 stars.
Each star has four pada or parts and 9 such parts are set up for one Rashi or division of Zodiac. Thus each part is of 30 degrees divided by 9 = 3.20 degree.
The following chart is essential for reference. Your Janma Rashi is in accordance with the position of star pada or part. For example, if your star is Krithika 1st part, it is Aries, and if it is Kritika 2nd part, it is Taurus, and so on.
This chart can give you a clear understanding of your Rashi. You would be able to know through your star which Rashi you belong to, and which is your Zodiac sign—and then select your gem accordingly.
Aries or Mesha
Ashwini 1,2,3,4 parts
Bharani 1,2,3,4 parts
Krittika 1st part
Taurus or Rishabha
Krittika 2,3,4 parts
Rohini 1,2,3,4 parts
Mrigashira 1,2 parts
Gemini or Mithuna
Mrigashira 3,4 parts
Ardra 1,2,3,4 parts
Punarvasu 1,2,3 parts
Cancer or Kataka
Punarvasu 4th part
Pushyami 1,2,3,4 parts
Aslesha 1,2,3,4 parts
Leo or Simha
Makha 1,2,3,4 partsPurva Phalguni 1,2,3,4 parts Uttara Phalguni 1st part Virgo or Kanya
Uttara Phalguni 2,3,4 parts
Hasta 1,2,3,4 parts
Chitra 1,2 parts
Libra or Tula
Chitra 3,4 parts
Swati 1,2,3,4 parts
Vishakha 1,2,3 parts
Vishakha 4th part
Anuradha 1,2,3,4 parts
Jyeshta 1,2,3,4 parts
Sagittarius or Dhanur
Moola 1,2,3,4 parts
Purvashada 1,2,3,4 parts
Uttarashada 1st part
Capricorn or Makara
Uttarashada 2,3,4 parts Shravana 1,2,3,4 parts
Dhanishta 1,2 parts
Aquarius or Kumbha
Dhanishta 3,4 parts
Satabhisha 1,2,3,4 parts
Purvabhadra 1,2,3 parts
Pisces or Meena
Purvabhadra 4th part
Uttarabhadra 1,2,3,4 parts
Revati