gem factory Chiang Mai

> if the gem factory near the handicraft centres around Chiang Mai would be a more reliable place for buying jewelry.

Reliable yes, cheap no. There's a huge one on the Sankamphaeng Road, half way to Bo Sang on the right.

I think you can often do better in regular jewellery shops in department stores. Airport Plaza has a bunch.

I'd vouch for it, bought a nice sapphire ring there for a former g/f.
appraisal elsewhere confirmed the price was fair.

It's worth a look even if you don't buy. It's a beautiful shop, and you can watch them prepping the stones and the metals there.
pretty sure it's the same one Chanchao mentions, touted as the biggest gem gallery in the world.