She stole it.......how can i get it back???

i had this vey expensive loreal lip-gloss......today,my friend asked for it and after she used it......she kept asking me where i got it from and all those other details.......then after a wile she asked for it again,i gave it to her and then went away......i saw her giving it to my other cl mate "b",and then she took it back from"b" and then went and kept it in her bag....later i asked for it again and she started pretending tht she doesnt have it and she started blaming friend"b" tht she has stolen it even though i know friend b hasnt stolen it......i have talked to the thief frind "a" twice i even told her tht i know she has it and she has to buy me a new one but she wont agree or admit tht she has it wht shud i do now......i need it or els my mom wud kill me.......plz give me serious help.....and plz remember tht friend b hasnt stolen it friend a has.....for sure !!!!!!