A.D. Leveridge Gauge – From their website:

“An indispensable tool for every jeweler, this instrument will measure inside and outside diameter, depth and thickness of gemstone, pearls–mounted or loose–to an accuracy of better than 1/10 mm. It includes a complete reference book for estimating the actual weight of all shapes of diamonds of all sizes, an attachment for measuring mounted stones with high settings, and an elegant carrying case.”