PHYSICAL PROPERTIES OF GEMSTONES

   

Gemstones cannot typically be classified simply by their color, as the same gem may occur in many shades.  To properly identify a gem it is necessary to look to other characteristics of the material.  A brief list of the most commonly used physical characteristics appears below.  

     To view information on a particular stone either scroll down through the list below, or click on one of the links to the left.  Additional information on the gem properties is included at the end of the list, as is a list of links to other sites.  

 

AMETHYST
Chemical Formula	SiO2	Hardness	7
Specific Gravity	2.6 - 2.7	Refractive Index	1.54 - 1.55
Amethyst is a type of Quartz.  The purple color is the result small quantities of iron or manganese compounds in the crystal.

AQUAMARINE
Chemical Formula	Be3Al2SiO6	Hardness	7.5 - 8
Specific Gravity	2.6 - 2.8	Refractive Index	1.57 - 1.58
Aquamarine is a type of Beryl, placing it in the same mineral group as emerald and heliodor.  Its characteristic light blue color is very similar to blue topaz, tourmalines, and peridots.

BERYL
Chemical Formula	Be3Al2SiO6	Hardness	7.5 - 8
Specific Gravity	2.6 - 2.8	Refractive Index	1.57 - 1.58
Beryl is a mineral group including Aquamarine, Emerald, and Heliodor.  The characteristic colors of these gems is the result of trace elements in the crystals.

CITRINE
Chemical Formula	SiO2	Hardness	7
Specific Gravity	2.6 - 2.7	Refractive Index	1.57 - 1.58
Citrine is a type of Quartz with a yellowish hue.  Most citrine is actually Amethyst which has been heat treated to produce the distinctive color.

EMERALD
Chemical Formula	Be3Al2SiO6	Hardness	7.5 - 8
Specific Gravity	2.6 - 2.8	Refractive Index	1.57 - 1.58
Emerald is a form of Beryl.  The green color is caused by small amounts of chromium or vanadium.  Emerald is among the most prized of gems, with the darker greens being the most rare.

GARNET
Chemical Formula	X3Y2Si3O12	Hardness	6.5 - 8.5
Specific Gravity	3.5 - 4.3	Refractive Index	1.78 - 1.89
Garnet is actually a group of minerals with closely related chemical and physical properties.  The "X" element is typically Mg, Fe, or Ca.  The "Y" element is typically Al, but may also be Fe3+ or Cr.  A bright red variation, Pyrope, has the chemical formula Mg3Al2Si3O12.

PERIDOT
Chemical Formula	X2SiO4	Hardness	6.5 - 7
Specific Gravity	3.2 - 4.2	Refractive Index	1.63 - 1.67
Peridot is the most desired member of the Olivine group.  Color ranges from yellow-green to olive-green.  The "X" component may be Mg or Fe.

RUBY
Chemical Formula	Al2O3	Hardness	9
Specific Gravity	3.9 - 4.1	Refractive Index	1.76 - 1.77
Ruby is one of the two varieties of Corundum, the other variety being Sapphire.  Rubies are easily confused with red Spinel.  Synthetic Corundum [both Ruby and Sapphire] is also available.

SAPPHIRE
Chemical Formula	Al2O3	Hardness	9
Specific Gravity	3.9 - 4.1	Refractive Index	1.76 - 1.77
Sapphire is one of the two varieties of Corundum, the other variety being Ruby.  Sapphire includes all colors of Corundum except for red stones, which are referred to as Ruby.  Sapphire is the most highly valued of the blue gemstones.

SPINEL
Chemical Formula	MgAl2O4	Hardness	7.5 - 8
Specific Gravity	3.5 - 3.7	Refractive Index	1.71 - 1.74
Spinel occurs in a wide variety of colors, the most popular being dark red.  Red Spinel is very difficult to distinguish from Ruby, requiring hardness or diffraction tests to be certain.

TOPAZ
Chemical Formula	Al2SiO4X2	Hardness	8
Specific Gravity	3.4 - 3.6	Refractive Index	1.61 - 1.63
Topaz is an aluminum silicate material which occurs naturally in a number of colors.  Natural stones may also be heat treated to yield pink, blue, and purple stones.  Citrine [Quartz] may easily be confused with Topaz, although the latter is more valuable.  The "X" component may be F or OH.

TOURMALINE
Chemical Formula	XY3Al6B3Si6(OH)4	Hardness	7 - 7.5
Specific Gravity	3.0 - 3.3	Refractive Index	1.62 - 1.65
Tourmaline occurs across the entire spectrum of gemstone colors, and may even be multicolored.  As with Garnet, Tourmaline is actually a group of closely related group of minerals.  The "X" component is Na or Ca.  The "Y" component may be Mg, Li, Al, or Fe2+.

SPECIFIC GRAVITY

     The specific gravity of a substance is a measure of its density [weight per given volume] as compared to water.  A substance with a specific gravity of 1.00 is exactly as dense as water.  A higher specific gravity means that the substance is more dense than water, while a specific gravity of less than 1.00 means that the substance is less dense than water. 

     Most gemstones have a specific gravity of between 3 and 4, thus are  three to four times denser than water.  The value of a gemstone is not typically determined by its specific gravity.  This measure is useful primarily in distinguishing between different types of minerals with similar optical characteristics.

HARDNESS

     If one substance will scratch another, the first substance is said to be the harder of the two.  A subjective scale to rate the relative hardness of mineral specimens was developed by Frederick Mohs in the early 1800's.  This scale, named for its creator, is still used today.  The Mohs scale ranks minerals on a scale of one to ten, based on the ability of the higher-rated materials to scratch the lower-rated materials.   The minerals used in the Mohs scare are:

Mohs Number	Mineral	Examples
1	Talc
2	Gypsum
2.5		Fingernail
3	Calcite	Copper Penny
3.5		Adamite
4	Fluorite
4.5		Steel Nail
5	Apatite
5.5		Glass
6	Feldspar
6.5		Steel File
7	Quartz
7.5		Beryl
8	Topaz
9	Corundum	Ruby
10	Diamond

REFRACTIVE INDEX [RI]

     The refractive index is a measure of the degree to which light is refracted [bent] as it through a particular stone. Light rays are deflected when they pass  from on air [or other medium] into a gemstone, and the measure of this deflection can be used to identify the gem material.  

     The refractive index is also used in determining how to shape a gemstone.  If the RI is known, the gem cutter can adjust the angles at which facets are ground onto the stone to maximize the effect of reflected and refracted light within the stone.