Matrimony Ring and Band recipes are listed below.

[Recipe for Matrimony Band and Ring]  
  ・Matrimony  Band
    Crafting Skill:    Goldsmithing
    Necessary Crystal:  Fire Crystal or Inferno Crystal
    Necessary Materials:  2 Platinum Ingots and 5 Diamonds

  ・Matrimony  Ring
    Crafting Skill:  Goldsmithing
    Necessary Crystal:  Fire Crystal or Inferno Crystal
    Necessary Materials:  2 Platinum Ingots and a Diamond

*You  can  engrave  your  name  on  the  ring  if  you  use  Inferno  Crystal  instead  of  Fire  Crystal  to  synthesize  the  ring

The rings can be engraved with your name later.  Recipe  below  explains  how  to  engrave  your  name  on  the  Matrimony  Ring  or  Band.

[Recipe to engrave names on the rings]
  ・Matrimony  Ring  or  Band
    Crafting Skill:  None
    Necessary Crystal:  Cyclone Crystal
    Necessary Item:  Matrimony Ring or Band

*If  Matrimony  Ring  or  Band  already  has  a  name  engraved,  then  synthesizing  them  using  Cyclone  Crystal  will  overwrite  the  name

Matrimony  Ring  and  Band  can  also  be  purchased  from  Matrimonial  Coffer.

[Matrimonial Coffer and Celebratory Chest]
http://support.na.square-enix.com/faqarticle.php?kid=51794&ret=main&id=20&la=1