Specific Gravity

Specific gravity (SG) tells you how heavy a gemstone is compared to an equal volume of water. For example, Ruby has a specific gravity of 4.0, which means it's four times denser or heavier than water. At the same physical volume, Ruby would weigh four times as much as water.

Gemstones come in many different densities and this is important when it comes to physical size. Two gemstones can have exactly the same carat weight, but can look noticeably different in size if their specific gravities differ. For example, a gemstone with one of the lowest SG is Amethyst (Quartz family) - 2.65, and one of the highest SG belongs to Zircon (high-type) ~ 4.7. If we compare 1 carat Amethyst vs Zircon and assume that cut proportions are exactly the same, then Amethyst would face-up 46% larger. See the difference:

How is this possible? It's because Amethyst is less dense, less heavy, and you need more of it to get to the same weight. Conversely, Zircon will have more of its mass concentrated in a smaller volume, resulting in smaller overall size.

Key thing to remember: At the same carat weight, a denser stone will be smaller in physical size than a less dense stone.

For most gemstones, specific gravity is expressed as a range rather than a single exact number. This is because natural gemstones aren't perfectly uniform in chemical composition and can include a certain degree of impurities and inclusions. It's common and just reflects the normal variation found in natural specimens.

Gemstone Specific Gravity Chart: 36 Gemstones