Gem testers have limited use. Those that use the heat transfer rate ie the Presidum can differentiate between Cubic Zirconia, sapphire and Topaz but not necessarily between Moissanite and Diamond.
A diamond tester can not differentiate between natural and lab grown diamonds.
If you are in the business of diamond dealing / trading / selling it would be much better to undertake gemology training so you can use a loupe or microscope to understand diamond structure, inclusions and colour grading. Understanding that Moissanite, for eg, is doubly refractive would stop you mistaking Moissanite for diamond even though the “tester” might indicate otherwise.
Lab diamonds are a whole other minefield, they should be inscribed as such on the girdle. There is no basic / general equipment that can differentiate between lab grown diamond and natural.
No that's not actually accurate as we really can tell by using the gemlogis tool.
Andygems, your profile lists you as trade. Is that an error?