for the larimar drive passed Barahona hang a right up the mountains . you get to the larimar mines. pay the guard 20 pesos
he wil let you pick up as much larimar as you can cary. For the amber drive passed santiago direction sosua to the amber mines. There amber will cost you a little more than the larimar.
The gift shops in Bayahibe are pretty bad, and I wouldn't trust them. Your best bet is to go to the jewellery store in Altos de Chavon, which has good high quality stuff.