Hydrostatic Weighing with a Modern Electronic Balance

Summary
Learn how to measure gemstone specific gravity using a modern single-pan electronic balance. This simple hydrostatic weighing method ensures fast, accurate results for stones under 3 carats using distilled water, a wire cage, and a lightweight beaker.

Introduction

Hydrostatic weighing is one of the most reliable methods to measure the specific gravity (SG) of gemstones. Modern single-pan electronic balances make this process faster, more precise, and easier than the old twin-pan systems.

You can even measure small gemstones below 3 carats accurately.

If you already use a gemological microscope or refractometer, this method is a perfect next step to confirm your findings.

What You Need

To perform accurate hydrostatic weighing, prepare the following:

• A modern electronic carat balance
• A lightweight beaker filled to the top with distilled water
• A simple stand with a thin wire attachment
• A small wire cage or basket to hold the stone

Tip: You can even use the lid of an aerosol can or a 35 mm film canister as your beaker. Make sure the wire cage doesn’t touch the sides.

Step-by-Step Guide

Step 1: Weigh in Air

Place the gemstone on the balance pan.
Record its weight in air (Wa).

Step 2: Set Up Water Beaker

Remove the stone and place the water-filled beaker below the balance pan.
Ensure the wire cage is fully submerged and not touching the sides.
Zero the balance.

Step 3: Weigh in Water

Gently lower the gemstone into the cage.
Make sure no water splashes onto the pan.
Record the weight in water (Ww).

Step 4: Calculate Specific Gravity

Use this simple formula:

SG = Wa ÷ (Wa – Ww)

Example:
If a gem weighs 2.50 ct in air and 2.00 ct in water:
Loss in water = 0.50
SG = 2.50 ÷ 0.50 = 5.00

Why This Method Matters

Hydrostatic weighing helps you:

• Distinguish look-alike gemstones
• Confirm identity and authenticity
• Improve accuracy in small stone testing

You’ll be amazed how small SG differences reveal major gem distinctions.

Learn more about Specific Gravity in Gemology.

FAQ

1. Can I use tap water instead of distilled water?
It’s better to use distilled water to avoid mineral contamination that affects readings.

2. How precise should the balance be?
A 0.001 ct accuracy balance is ideal for gemstones under 3 carats.

3. Can this method detect synthetic stones?
Not directly, but it helps identify inconsistencies in SG that may indicate treatments or synthetics.