To obtain approximate weights of the following shapes of colored stones multiply by the following measurements to millimeters. A listing of the specific gravities is below.

Round Faceted = ...................Diameter x diameter x depth x SG x .0018

Oval Faceted = .......................Diameter x diameter x depth x SG x .0020

Emerald Faceted = .................Length x width x depth x SG x .0025

Rectangle Faceted = .............Length x width x depth x SG x .0026

Square Faceted = ...................Length x width x depth x SG x .0025

Marquise Faceted = ...............Length x width x depth x SG x .0017

Pear Faceted = ........................Length x width x depth x SG x .0018

Heart Faceted = ......................Length x width x depth x SG x .0018

Table of Specific Gravities for Popular Gemstones

Zircon: ........4.32 - 4.70 Diamond: ................3.52 Conch Pearl: ............2.85

Almandite Garnet: 4.05 Peridot: ....................3.34 Turquoise: .................2.76

Ruby: ......................4.00 Jadeite: .....................3.34 Lapis Lazuli: .............2.75

Sapphire: ................4.00 Zoisite (tanzanite) ..3.35 Beryl Group: ............2.72

Malachite: ..............3.95 Diopside: .................3.29 Pearl: ..........................2.70

Rhodolite Garnet: 3.84 Spodumene: ............3.18 Quartz: .......................2.66

Pyrope Garnet: .....3.78 Andalusite: ...............3.17 Coral: .........................2.65

Chrysoberyl: .........3.73 Tourmaline: .............3.06 Iolite: ..........................2.61

Spinel: ....................3.60 Nephrite: ..................2.95 Opal: ..........................2.15

Source: http://www.yourgemologist.com/Charts%20and%20Graphs/coloredgemweights.html

Note: you can incorporate the constants in the diamond estimation table for emerald, pear and marquise to get a better estimate for those here