christine davidson lost her battle to cancer at an adelaide nursing home .
her diamond ring was last seen three weeks before she passed away .
the family has claimed a brazen thief is behind the missing ring .
the 61-year-old wanted to hand her ring down to her only granddaughter .
the family have launched an emotional public appeal to get her ring back .
a local jewellery store has offered a $ 1000 reward for anyone providing information that leads to finding the missing ring .