Gemstones
Amethyst was as expensive as ruby and emerald until the 19th Century, when Brazil’s large deposits were discovered. It was believed to prevent intoxication—amethystos means “not drunk” in ancient Greek. Today, as the most valued quartz variety, amethyst is in demand for designer pieces and mass-market jewelry alike, and its purple to pastel hues retain wide consumer appeal.
FACTS
Mineral: Quartz
Chemistry: SiO2
Color: Purple
Refractive Index: 1.544 to 1.553
Birefringence: 0.009
Specific Gravity: 2.66
Mohs Hardness: 7
Aquamarine’s name comes from the Latin for seawater and it was said to calm waves and keep sailors safe at sea. March’s birthstone was also thought to enhance the happiness of marriages. The best gems combine high clarity with limpid transparency and blue to slightly greenish blue hues. Like many beryls, aquamarine forms large crystals suitable for sizable fashioned gems and carvings.
FACTS
Mineral: beryl
Chemistry: Be3Al2Si6O18
Color: greenish blue, light in tone
Refractive index: 1.577 to 1.583
Birefringence: 0.005 to 0.009
Specific gravity: 2.72
Mohs Hardness: 7.5 to 8
Citrine is rare in nature. In the days before modern gemology, its tawny color caused it to be confused with topaz. Today, its attractive color, plus the durability and affordability it shares with most other quartzes, makes it the top-selling yellow-to-orange gem. In the contemporary market, citrine’s most popular shade is an earthy, deep, brownish or reddish orange.
FACTS
Mineral: Quartz
Chemical composition: SiO2
Color: Yellow to orange to orangy red
Refractive index: 1.544 to 1.553
Specific gravity: 2.66 (+0.03/-0.02)
Mohs hardness: 7
Gem experts differ on the degree of green that makes one stone an emerald and another stone a less-expensive green beryl. Most gemologists, gemological laboratories, and colored stone dealers call a stone green beryl when its color is “too light” for it to be classified as emerald. Even among that group, however, there’s a difference of opinion about what’s considered “too light.”
FACTS
Mineral: Beryl
Chemistry: Be3Al2Si6O18
Color: Vibrant green
Refractive Index: 1.577 to 1.583
Birefringence: 0.005 to 0.009
Specific Gravity: 2.72
Mohs Hardness: 7.5 to 8
Red garnets have a long history, but modern gem buyers can pick from a rich palette of garnet colors: greens, oranges, pinkish oranges, deeply saturated purplish reds, and even some blues. Red garnet is one of the most common and widespread of gems. But not all garnets are as abundant as the red ones. A green garnet, tsavorite, is rarer and needs rarer rock chemistries and conditions to form.
FACTS
Mineral: Garnet group
Chemistry:
Almandine- Fe3Al2(SiO4)3
Andradite- Ca3Fe2(SiO4)3
Grossular- Ca3Al2(SiO4)3
Pyrope- Mg3Al2(SiO4)3
Rhodolite- (Mg, Fe)3Al2(SiO4)3
Spessartine- Mn3Al2(SiO4)3
Color: All colors
Refractive index: 1.714-1.888
Birefringence: None
Specific gravity: 3.47-4.15
Mohs harness: 6.5-7.5
Jade has its cultural roots in the smoke-dimmed caves and huts that sheltered prehistoric humans.
Around the world, Stone Age workers shaped this toughest of gems into weapons, tools, ornaments, and ritual objects. Their carvings invoked the powers of heaven and earth and mystic forces of life and death.
FACTS
Mineral: Jadeite and Nephrite
Chemistry: NaAlSi2O6 and Ca2(Mg,Fe)5Si8O22(OH)2
Color: Green, white, orange, yellow, lavender, black
Refractive index: 1.666 to 1.68 (+/-0.008) and 1.606 to 1.632 (+0.009, 0.006)
Birefringence: Usually not detectable
Specific gravity: 3.34 and 2.95
Mohs Hardness: 6.5 to 7 and 6 to 6.5
This ancient rock is an aggregate of several minerals. The three major minerals that comprise lapis are lazurite, calcite, and pyrite. The rock can also contain lesser amounts of diopside, amphibole, feldspar, or mica. Lazurite is the ingredient responsible for producing the gem’s most prized color—bright royal blue.
FACTS
Mineral: Rock
Chemical composition: An aggregate
Color: Greenish blue to violetish blue
Refractive index: 1.500, sometimes 1.67
Birefringence: None
Specific gravity: 2.75 (+/- 0.25)
Morganite’s subtle color is caused by traces of manganese. Because morganite has distinct pleochroism —pale pink and a deeper bluish pink—it’s necessary to orient the rough carefully for fashioning. Strong color in morganite is rare, and gems usually have to be large to achieve the finest color.
FACTS
Mineral: Beryl
Chemical composition: Be3Al2Si6O18
Color: Pink to orange-pink
Refractive index: 1.583 to 1.59
Birefringence: 0.007 to 0.008
Specific gravity: 2.8 to 2.91
Mohs hardness: 7.5 to 8
The ancient Egyptians mined peridot on the Red Sea island of Zabargad, the source for many large fine peridots in the world’s museums. The Egyptians called it the “gem of the sun.” Today this gem is still prized for its restful yellowish green hues and long history. Large strongly-colored, examples can be spectacular, and attractive smaller gems are available for jewelry at all price points.
FACTS
Mineral: Olivine
Chemistry: (MgFe)2SiO4
Color: Yellowish green
Refractive index: 1.65 to 1.69
Birefringence: 0.035 to 0.038
Specific gravity: 3.34
Mohs Hardness: 6.5 to 7
Rubies can command the highest per-carat price of any colored stone. This makes ruby one of the most important gems in the colored stone market. In its purest form, the mineral corundum is colorless. Trace elements that become part of the mineral’s crystal structure cause variations in its color. Chromium is the trace element that causes ruby’s red color.
FACTS
ACTS
Mineral: Corundum
Chemical composition: Al2O3
Color: Red
Refractive index: 1.762 to 1.77
Birefringence: 0.008 to 0.01
Specific gravity: 4 (+/- 0.05)
Mohs Hardness: 9
Besides blue sapphire and ruby, the corundum family also includes so-called “fancy sapphires.” They come in violet, green, yellow, orange, pink, purple, and intermediate hues. Some stones exhibit the phenomenon known as color change, most often going from blue in daylight or fluorescent lighting to purple under incandescent light. Sapphires can even be gray, black, or brown.
FACTS
Mineral: Corundum
Chemistry: Al2O3
Color: Every color but red
Refractive index: 1.762 to 1.77
Birefringence: 0.008 to 0.01
Specific gravity: 4
Mohs Hardness: 9
Until recently, spinel was an underappreciated gem with little consumer recognition. Increasing demand for ruby alternatives rekindled appreciation for spinel’s rich red color and history. In ancient times,
southeast Asia’s mines yielded exceptional large spinel crystals, which became the treasured property of kings and emperors, often passing through many hands as spoils of war.
FACTS
Mineral: spinel
Chemistry: MgAl2O4
Color: red, orange, pink, purple, blue, black
Refractive index: 1.718
Birefringence: none
Specific gravity: 3.6
Mohs Hardness: 8
Topaz actually has an exceptionally wide color range that, besides brown, includes various tones and saturations of blue, green, yellow, orange, red, pink, and purple. Colorless topaz is plentiful, and is often treated to give it a blue color. Topaz is also pleochroic, meaning that the gem can show different colors in different crystal directions.
FACTS
Mineral: Topaz
Chemistry: Al2(F,OH)2SiO4
Color: Yellow, orange, brown, pink to red to purple red, blue, light green and colorless
Refractive index: 1.619 to 1.627
Birefringence: 0.008 to 0.01
Specific gravity: 3.53
Mohs Hardness: 8
Tourmaline’s colors have many different causes. It’s generally agreed that traces of iron, and possibly titanium, induce green and blue colors. Manganese produces reds and pinks, and possibly yellows. Some pink and yellow tourmalines might owe their hues to color centers caused by radiation, which can be natural or laboratory-induced.
FACTS
Mineral: Tourmaline
Chemistry: Elbaite Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)4 Dravite NaMg3Al6Si6O18(BO3)3(OH)4 Liddicoatite Ca(Li2Al)Al6Si6O18(BO3)3(OH)3F Chromedravite NaMg3Cr6Si6O18(BO3)3(OH)4
Color: All colors
Refractive index: 1.624 to 1.644
Birefringence: 0.018 to 0.04
Specific gravity: 3.06 (+0.20, -0.06)
Mohs Hardness: 7 to 7.5
Colorless zircon is known for its brilliance and flashes of multicolored light, called fire. These zircon properties are close enough to the properties of diamond to account for centuries of confusion between the two gems.
Zircon occurs in an array of colors. Its varied palette of yellow, green, red, reddish brown, and blue hues makes it a favorite among collectors as well as informed consumers.
FACTS
Mineral: Zircon
Chemistry: ZrSiO4
Color: Blue, red, yellow, orange, brown, green
Refractive index:
High: 1.925 to 1.984 (+/- 0.040)
Medium: 1.875 to 1.905 (+/- 0.030)
Low: 1.81 to 1.815 (+/-0.030)
Birefringence: 0 to 0.059 (low to high)
Specific gravity: 3.9 to 4.73
Mohs Hardness: 6 to 7.5 (low to high)