What specific gravity measures
Specific gravity is how heavy a material is compared with the same volume of water. A gem with an SG of 4.00 weighs four times as much as a drop of water of exactly its size. Because SG depends only on what a material is made of and how its atoms are packed, it is one of the properties gemmologists use to narrow down what a loose stone might be.
It is measured by weighing the stone twice: once in air and once suspended in water. The difference between the two weighings is the weight of the water the stone pushed aside, and SG is the weight in air divided by that difference. The specific gravity calculator on this site does that arithmetic and works out how far off the result could be from the scale’s resolution.
Where these figures come from
Every figure on the chart is copied from the facts panel on the GIA Gem Encyclopedia page for that gem. GIA prints SG in three ways, and the chart keeps each one as printed. Some gems have a value with a tolerance: ruby is 4.00 plus or minus 0.05, diamond 3.52 plus or minus 0.01, tourmaline 3.06 with 0.20 above and 0.06 below. Some have a range: garnet 3.47 to 4.15, zircon 3.90 to 4.73, pearl 2.60 to 2.85. And some have a single value with no tolerance printed: sapphire 4.00, spinel 3.60, topaz 3.53.
Where GIA gives a single value, the chart draws it as a single line and does not add a tolerance of its own. Natural stones do vary, and a careful reading of a real sapphire may not land on exactly 4.00, but inventing a margin GIA did not publish would put a number on the page with no source behind it. The uncertainty box lets you widen your own reading instead.
Why garnet and zircon cover so much of the chart
“Garnet” is not one mineral but a group of closely related ones, such as almandine, pyrope, spessartine, grossular and andradite, each with a different chemistry. GIA publishes one range for the whole group, and it is wide enough to overlap diamond, spinel, topaz and ruby. A reading inside the garnet range says almost nothing on its own.
Zircon is wide for a different reason. GIA describes high, medium and low types, whose properties differ, and its SG range spans all three. The refractive index chart on this site splits zircon into those types because GIA’s RI figures are given separately; its SG is given as one range, so it is shown as one here.
What a reading can and cannot tell you
SG narrows a list; it does not name a stone. Emerald and aquamarine share a figure of 2.72 because both are beryl. Ruby and sapphire share 4.00 because both are corundum. Amethyst and citrine are both quartz at 2.66. Jadeite and peridot are both printed at 3.34. In each case a second test, such as refractive index, a spectroscope or simply colour, does work that SG cannot.
A reading also only describes the stone that was weighed. Mounted stones cannot be tested this way, because the metal is weighed with them. Porous materials such as turquoise and some opal can take on water while submerged, which changes the second weighing. Composite and assembled stones, doublets and triplets, give an SG for the whole assembly. And a synthetic stone has essentially the same SG as its natural counterpart, so this test does not separate them.
Gems that are not on the chart
Moissanite, cubic zirconia, chrysoberyl, demantoid, tsavorite and coral have no facts panel in GIA’s encyclopedia, so they are left off rather than filled in from another source. That matters most for the two diamond simulants: if you are trying to rule out moissanite or cubic zirconia, this chart cannot do it. A thermal or electrical diamond tester, or a gemmologist, can.
The gemstone carat size chart and the gemstone weight calculator on this site use an SG for each species to convert size to weight. Those tools need one figure per species to do arithmetic; this chart shows GIA’s published figures with their tolerances so the spread is visible.
Taking a careful reading
Small stones are hard to weigh accurately: on a scale that reads to 0.01 carat, a half-carat stone displaces so little water that the SG can be uncertain in the first decimal place. Use the most precise scale available, make sure the stone is fully under water and not touching the basket’s sides, and remove air bubbles from its surface. A drop of detergent in the water helps. Then enter the uncertainty the specific gravity calculator gives you, and read the chart for every gem that range overlaps.