Moissantie testers are not suitable for testing stones under 5 points in weight (= the diameter of the top [table] of the stone is about 2.5mm diameter) - the current will 'spark' onto the setting of the stone and give false readings.
If you don't have a steady hand and good eyesight (or an eyeglass) you will have difficulty in testing stones under 10 points in weight (= the diameter of the top [table] of the stone must be nearly 3mm diameter).
Also, the stone must be clean. That means cleaning with a cotton wool bud and alcohol (wiping the stone on a used handkerchief isn't good enough!).