This was posted somewhere on the forum and I thought it very easy to read and understand so perhaps a few of the many that have written wanting to know how to do specific Gravity tests here you go....

There is also a formula to deal with iron stone/quartz specimens!

Here's a tried and tested formula with a little explanation along the way.

(1)  Air Weight in grams = (A) - Water Weight in grams = (W)

(2) (A) - (W) = Volume of specimen in cubic centimeters. Based on Archimedes principle.

(3) Density = (A)/(V) = (D) in grams/cubic centimeter

(4) The Formula!

Proportion of gold = (D - 2.65) x 19.3 
(19.3 - 2.65) x D

A worked example

Air weight is 8.7g
Water weight is 7.0g

(2) So Volume is 8.7 - 7.0 = 1.7 cubic centimeters

(3) So Density is 8.7 / 1.7 = 5.12 grams per cubic centimeter

(4) Proportion of gold = (5.12 - 2.65) x 19.3 = 47.67 = 0.5592 gold or 55.92%
(19.3 - 2.65) x 5.12 85.25

Now multiply Air weight by .5592 to give 4.9 grams of gold

 If you have an ironstone specimen, replace 2.65 (which is just the 'density' of quartz),with the density of your ironstone. To do this, find some ironstone (free of gold or quartz) around where you find the specimen, and find out its density following steps (1) to (3) above and plug it in. This is necessary with ironstone specimens for accuracy because different iron stones have different densities (not like say different colored quartz which is all 2.65g/cubic cm - iron stones ain't iron stones). For those less inclined to find the density of their particular ironstone, generally here in Victoria, the lighter brown 'lava' type ironstone usually has a density similar to quartz , so just stick with 2.65, but for the really heavy (and noisy!) black iron stones like say around Wedderburn, you could use 3.50 to 4.00. It disappoints me to see so many ironstone specimens ruined in acid baths just to find out the gold content, hope this info can save a few.

This looks long and complicated but its not when you get down to just following the steps.