Plus or minus one light, e.g. if it reads 14ct it might be 12ct or it might be 16ct but not this accurate above 18ct. OK if you want to know if an item is "low carat" or, possibly, "high carat" – otherwise, fairly useless.
KEE (MODEL M-509GM)
Click here to buy.
( click here if you already have one, and need a replacement probe)
AuRACLE AGT1 PLUS DELUXE by GemOro
Update: this one discontinued in 2022 due to problems with UK / EU electrical safety regulations.
…but we can get replacement probes to special order.
AuRACLE AGT3 by GemOro
Instead of a row of LED lights it has a digital display. It is not just having a digital display that makes the difference, it’s the way they have computerised the on-display instructions.
VERY OLD (OBSOLETE) AGT2
The GemOro AuRACLE AGT2 consisted of a probe, a testing plate, a box of electronics (not shown in the manufacturers pictures), connecting leads and a large fitted case. You connect it to your phone.