HI Teezer,
I looked at the Gem-A on Amazon ($549) and the Kassoy at its website ($109). I can’t see that they do anything different, both having scales that read to 0.01 divisions. What you would need to query their vendors on is what light source there is…is there a yellow LED or some kind of yellow filter and whether they come (I should hope) with a polarizing filter or not. You need the filter and a yellow sodium wavelength filter or a yellow light source. The shapes are close enough that I suspect they may even have the same internal parts, altho’ that’s only a guess. Moreover, I see about the same (similar?) instrument on ebay for about $60 and see no reason to go above that price…just, as I said, get a return privilege, so you can return any defective one for another…sooner or later you’ll get one that is calibrated right, even if the first is defective. It’s just a QC issue.
The filters are really not a must-have, but they are fun to play with and great short cuts to ID, esp. of parcels. I think there’s a Chelsea filter in the Hannemann set, so you may be able to return the Chelsea filter and save some bucks. What you really should get is Hannemann’s book, which I see is now just $38! You could get the book and a cheapie ebay refractometer for the price of the Kassoy, altho’ Kassoy is a very reliable dealer, so you pick.
As for bugging me, help, I’m talking about gemology and can’t shut up…LOL…ask away…-royjohn