The garnet is a gemstone for Rahu. Some points relevant to this stone are as under.
[21] RING / PENDANT OF GARNET & THE TIME OF MOUNTING OF GARNET (GOMED) IN THE METAL
The ring or pendant of garnet (Gomed) should be made with 7.0 to 8.0 grams of iron or Panch-dhatu (refer to chapter 14 of this book for the definition of Panch-dhatu).
The garnet can be mounted in the selected metal on any Wednesday morning. It is preferable if Swati Naxatra, Sat-tara Naxatra, or Adra Naxatra occur on this morning.
The ring or pendant should be made in such a way that maximum surface of garnet should touch the skin of the wearer.
[23] REQUIRED PRAYER [POOJA]
When the ring or pendant will be ready, then a Rahu yantra (a chart drawn on a small sheet of iron which represent Rahu) should be prepared on a small iron sheet, and prayer should start at 3.00pm to 6.00pm on any Wednesday. It is preferable if any one of the Naxatras like, Swati , Sat-tara or Adra Naxatra falls on this day. A total of 18,000 mantras should be chanted on the ring or pendent.
The mantra for Rahu is as under.
"OM ANG HRIM RAHVE NAMAH"
For the remaining 23 points, you can contact us or you can refer to our book “Astrological Science... Your Birthstones and You".