Dealers in precious stones

Just a favour of those more knowledgable than myself (of which you number many). The mother in law bought a diamond, ruby and sapphire in the 80's (bad investment I know) and now wants to sell them. She is in a regional town and the local jeweller there had recently valued the diamond at $25,000 ( she bought it for $20,000) . Anyway, as soon as she mentioned selling it he said that was for insurance only and offered her $8000. I have no idea at all whether this is fair value and I have asked her to send me the details of the stones so I can research further but in the meantime what is the best method to use to sell these stones (she is in Queensland). Any advice to pass on to her would be greatly appreciated.

Cheers, Spotty