An Electronic Scale can be used to measure the specific gravity
of gemstones and minerals. As with most hydrostatic devices, you simply weigh the stone "in air" and then weigh it in water. A typical scale intended for such purpose has a "weigh below" hook
on the underside of the scale from which to a "gem holder" in a container of water. While this works for a weighing of the stone in water, it is an awkward operation - scale must be elevated on a stack of books or the like, and the user must work in a confined space underneath it.
Typical density determination kits for lab balances
cost in the $500 range and they are designed for just one particular model. Mineralab has instead developed an affordable user-friendly "above the scale" underwater weighing process referred as a Universal Specific Gravity Kit
. Just weigh the stone "in air" on the integrated weighing platform the scale, then without changing any setup, weigh the stone in water by placing it in the underwater weighing pan of the gem holder suspended above the platform. Then plug those two values into the specific gravity equation
to obtain the specific gravity of your stone. It's that easy and it takes only a few seconds!
For those who need accurate specific gravity measurements on small, lightweight gemstones, results are limited to the readability of the scale
. The specifications on the scale you choose needs to offer the weighing capacity
and more importantly good readability of the scale
enabling it to detect the lightest weight as possible. We recommend the minimum readability down to 0.001 to 0.002 grams (0.005 to 0.01 carats) for determining specific gravity of gem stones. Many people purchase the low cost Acculab Vicon Series Milligram Scales
while others opt for the top of the line Sartorius GC2502 Milligram Balance
. If money is no object, the
balance should read down to 0.0002 grams / 0.001 carats like the Sartorius GD603
. or Sartorius GC1603P
. Refer to this Table
for details on how accurate and sensitive a particular scale will be for specific gravity measurements.
The photo below shows the components of the Mineralab Universal Specific Gravity Kit
, and the image above since shown using this affordable density determination kit with the Acculab VIC-303 Electronic Scale
. A beaker of water sits within a spill-resistant support independent of the scale itself, while a lightweight gem holder rests on the scale's weighing platform, with the weighing pan suspended underwater. You can take a look at a real example determining the specific gravity of a piece of aluminum here
The components of the Mineralab Universal Specific Gravity Kit include the following:
- Lightweight Gem Holder Support and Underwater Weighing Pan
- Independent, Spill-Resistant, Adjustable Beaker Support
- Beaker for filling with water - 50 to 100ml capacity
- Aluminum Test Weight whose SG = 2.70
- Stainless Steel Tweezers
- Complete instructions (although this method is very simple). Step by step instructions are also shown here.