You’ve probably held a gemstone in your hand and wondered, "Is this actually real?" Maybe you bought a "ruby" at a flea market, or you found a clear crystal while hiking and you’re praying it’s a diamond (spoiler: it’s usually quartz, but we can dream).
The problem is that visual identification is notoriously tricky. A red stone could be a Ruby, a Spinel, a Garnet, or a piece of colored glass. Your eyes can be fooled. Physics, however, cannot.
This is where Specific Gravity (SG) comes in. It is the great equalizer—the lie detector of the gem world. In my years of collecting and sorting, this single test has saved me more money and embarrassment than any fancy refractometer ever could. And the best part? You don't need a laboratory. You just need your kitchen counter.
WHY SPECIFIC GRAVITY IS THE "LIE DETECTOR" FOR GEMSTONES
Specific Gravity is essentially a measure of density—how heavy a stone is relative to its size. Every gemstone species has a unique chemical signature that dictates its density. A diamond will always be denser than a piece of glass of the exact same size.
The Kitchen Scale Skepticism
You might be thinking, "Patricia, can I really do scientific gemology with the same scale I use to weigh flour?" The answer is a resounding yes, provided you understand the limitations. While a professional gem lab uses hydrostatic balances worth thousands of dollars, a decent digital kitchen scale can get you within 0.01 to 0.05 of the correct specific gravity. That is often close enough to distinguish a Topaz from a Quartz or a Diamond from a Cubic Zirconia.
Real vs. Fake: The Density Truth
This test is particularly ruthless when it comes to synthetics and simulants. Glass and plastic are the most common "fakes" you’ll encounter. Plastic is incredibly light (low SG), while lead glass can be heavy but usually doesn't match the specific target of the stone it's mimicking. When you run the numbers, the math reveals the imposter immediately.
THE DIY SPECIFIC GRAVITY KIT: WHAT YOU NEED (THAT YOU ALREADY OWN)
You don't need to buy a $500 kit from a gemology supplier. You can MacGyver this right now with items in your drawers.
The Scale: Why 0.01g Accuracy Matters
This is the only non-negotiable. Your standard diet scale that weighs in 1-gram increments is useless here. You need a scale that reads to at least 0.01 grams (hundredths of a gram). If you try this with a less sensitive scale, your math will be so far off that a diamond will look like a brick.
The Water Container
You need a plastic cup or a small beaker. It needs to be deep enough to fully submerge your stone without the water overflowing.
The Suspension Mechanism
You need a way to dangle the stone in the water. I use a paperclip bent into a hook and a thin piece of thread or fishing line. The thinner the line, the better, as the line itself adds a tiny bit of weight that we want to minimize.
The "Water Bridge" Setup
This is the part that confuses people, so visualize this carefully. You are not putting the cup of water on the scale. You are creating a bridge over the scale.
Bridge Construction
Place two identical objects (like two soup cans or two stacks of books) on either side of your scale. Place a flat ruler or board across them. The cup of water sits on this bridge. The scale sits underneath the bridge. The goal is to hang the stone from the scale into the water without the stone touching the bottom or sides of the cup.
Patricia's Pro-Tip:
"Stability is everything. If your 'bridge' is wobbly, or if your ceiling fan is blowing the thread around, your numbers will jump all over the place. Turn off the fans and hold your breath when you take the final reading."
THE PROCESS: STEP-BY-STEP
Now that we have our rig, let's do the math. We need two numbers: the weight of the stone in the air, and the apparent weight of the stone when it is suspended in water.
Step 1: The Dry Weight (Weighing in Air)
Place your stone directly on the scale platform. Wait for the numbers to settle. Write this number down. This is your variable "A."
Step 2: The Wet Weight (The Suspension Method)
This is the tricky part. Place a small container of water on the scale and hit "tare" (zero it out). Now, tie your stone to the thread. Gently lower the stone into the water. The stone must be fully submerged but must not touch the bottom or sides of the cup. Hold it steady. The scale will register a weight. This is actually the weight of the water displaced by the stone (Archimedes' principle). Write this number down. This is variable "B."
The Magic Formula
The formula is simple division.
Specific Gravity (SG) = Dry Weight ÷ Wet Weight (Displaced Weight).
Calculation Example: Testing a Quartz Crystal
Let’s say you have a clear crystal.
Dry Weight: 15.45 grams.
Wet Weight (Displacement): 5.83 grams.
The Math: 15.45 divided by 5.83 equals 2.65.
Looking at the charts, 2.65 is the exact specific gravity of Quartz. You have successfully identified your stone.
DATA & REFERENCE: YOUR IDENTIFICATION CHEAT SHEETS
To make this actionable, I’ve compiled the specific gravity ratings for the stones I get asked about most often.
Cheat Sheet: Common Gemstone Specific Gravity
| Gemstone | Specific Gravity (SG) Range | Common Confusion |
| Amber | 1.05 - 1.09 | Floats in salt water (very light) |
| Opal | 1.98 - 2.25 | Light feel, often confused with plastic |
| Quartz (Amethyst/Citrine) | 2.65 | The standard baseline for most stones |
| Emerald | 2.72 (avg) | Lighter than diamond or ruby |
| Tourmaline | 3.06 (avg) | Often confused with Emerald |
| Diamond | 3.52 | Dense and heavy for its size |
| Garnet (Almandine) | 3.90 - 4.30 | Very heavy, often confused with Ruby |
| Cubic Zirconia (CZ) | 5.60 - 6.00 | Extremely heavy "fake" diamond |
| Gold (24k) | 19.3 | Unmistakably heavy |
The Showdown: Look-Alike Comparisons
When you are trying to distinguish between two specific stones that look identical, this comparison table is your best friend.
| Comparison | Stone A (SG) | Stone B (SG) | The Verdict |
| Diamond vs. CZ | Diamond (3.52) | Cubic Zirconia (~5.80) | CZ is nearly twice as heavy. If the stone feels oddly heavy for its size, it's likely CZ. |
| Ruby vs. Red Garnet | Ruby (4.00) | Red Garnet (~3.80 - 4.30) | This is tricky as they overlap. However, if you get a reading over 4.10, it is almost certainly Garnet, not Ruby. |
| Aquamarine vs. Blue Topaz | Aquamarine (2.72) | Blue Topaz (3.53) | Topaz is much denser. The "heft" test alone can often separate these two. |
TROUBLESHOOTING: WHY IS MY RESULT WRONG?
You did the math, and you got a result of 11.2, which isn't on any chart. Don't panic. You likely fell victim to physics.
The "Bubble Trouble": How Surface Tension Ruins Your Test
Air bubbles are the enemy. When you submerge a rough stone or a faceted gem into water, tiny air bubbles love to cling to the surface. These bubbles act like little life jackets, lifting the stone up and messing with your "Wet Weight."
The Fix
Before you weigh the stone in water, rub a tiny drop of dish soap onto the stone, or put a drop of soap into the water. This breaks the surface tension. When you lower the stone in, give it a little shake to knock off any clinging bubbles.
Can I Test Mounted Gemstones?
This is the most common question I get: "Can I test the diamond in my engagement ring?" The short answer is no.
The metal setting (gold, platinum, silver) has its own weight and density. The scale cannot separate the weight of the gold from the weight of the stone. If you weigh the whole ring, you get an average density of the metal and stone combined, which tells you nothing about the stone itself.
Speaking of metals, if you are dealing with jewelry that involves metal settings, knowing your metals is just as important as knowing your stones. For example, distinguishing silver plate from solid silver requires a different set of eyes; check out EPNS vs. Sterling Silver: How to Identify & Test Real Silver at Home for a deep dive on that topic.
ADVANCED TIPS FOR THE OBSESSED
Once you master the basic dunk-test, you can refine your accuracy.
Tap Water vs. Distilled Water
Technically, tap water contains minerals that change its density slightly. For rough identification, tap water is fine. But if you are trying to distinguish between two very similar stones (like Tourmaline and Topaz), use distilled water at room temperature for the most accurate "Wet Weight."
Understanding "Heft"
After doing this enough times, you develop a "feel" for density, known as heft. This is the manual way to estimate density. If you bounce a piece of plastic in your hand, it feels empty. If you bounce a piece of tin-rich pewter or gold, it feels significant. Specific gravity puts a number to that feeling.
FREQUENTLY ASKED QUESTIONS ABOUT SPECIFIC GRAVITY
What is the easiest gemstone to identify with this test?
Cubic Zirconia (CZ). Because it is so much denser than almost any natural stone it mimics, it sticks out like a sore thumb on a specific gravity test. If you have a "diamond" that tests at 5.8 SG, it is definitely a simulant.
Does the size of the stone matter?
Yes. Very small stones (under 1 carat) are difficult to test on a kitchen scale because the margin of error is too high. The heavier the stone, the more accurate your result will be because the displacement weight is easier for the scale to register.
Can I use this test for rough rocks found in the river?
Absolutely. In fact, it works better on rough rocks because they are usually larger. Just scrub the dirt off first; dirt adds weight but has a different density, which can skew your results.
The Final Takeaway
You don't need a PhD in geology to know what's in your pocket. With a $20 scale, a cup of water, and a little bit of patience, you can pierce the veil of mystery surrounding your gemstone collection. So, go raid your jewelry box, grab that mysterious rock from your window sill, and let physics tell you the truth.
