Rerefernsi

2. WHAT IS A GEM
• Some gems – like pearls, amber, coral, and ivory – are
Organic. But most gems are minerals : natural, inorganic
materials with specific chemical compositions and a
characteristic structure.
• Already know the name of some minerals commonly
found in jewelry store : topaz, sapphire, emerald, and
ruby, for example.

3. • A Gemstone or gem (also called a precious or
semiprecious stone, a fine gem, or jewel) is a pieces of
mineral, which, in cut and polished form, is used to make
jewelry or other adorment.
• Gemology – the studi og gems

4. This Cartier bracelet from 1927 features a 47.07 ct cabochon sap- phire between
two pear-shaped diamonds weighing 8.60 and 9.27 ct. The sapphire is accompanied
by two identification reports stating a Burmese origin, with no indications of
heating. In Novem- ber 2019, the bracelet sold at Sotheby’s in Geneva for nearly
$6.3 million. Courtesy of Sotheby’s.
Gemstone classics from sources old and new: A 2.50 ct
Colombian emerald ring, courtesy of Ronald Ringsrud; a 7.04
ct Madagascar sap- phire, courtesy of Mayer & Watt; and an
excep- tional 4.10 ct concave- facet ruby from Winza,
Tanzania, courtesy of Mark Gronlund. Photo by Robert
Weldon/GIA.

5. The name “sapphire” can also apply
to any corundum that’s not ruby,
another corundum variety.
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.
Mineral: Corundum
Chemistry: Al2O3
Color: Every color but red
Refractive index: 1.762 to 1.770
Birefringence: 0.008 to 0.010
Specific gravity: 4.00
Mohs Hardness: 9
SAPPHIRE

6. Sapphire comes in a variety of colors. This rare 1,126.00-ct.
corundum crystal’s pinkish orange color is called padparadscha,
named after the lotus flower. - © GIA & Tino Hammid

7. Figure. A 245.12 ct sapphire crystal from Kataragama,
Sri Lanka, measuring 5.70 × 2.15 × 1.85 cm. Photo by
Robert Weldon/GIA; courtesy of Brent Lockhart.
Figure. Rough bipyramidal sapphire crystal from Sri Lanka,
weighing 10.4 grams (51.70 ct). Photo by Robert Weldon/GIA;
courtesy of William Larson, Pala International.

8. Ruby is the most valuable variety of the
corundum mineral species, which also
includes sapphire.
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.
Mineral: Corundum
Chemical composition: Al2O3
Color: Red
Refractive index: 1.762 to 1.770
Birefringence: 0.008 to 0.010
Specific gravity: 4.00 (+/- 0.05)
Mohs Hardness: 9
RUBY

9. Rough and cut ruby: The crystal measures 23.4 mm in length, and the faceted stone weighs 5.22 ct.
The cut stone carries four different identification reports, with the country of origin listed as either
Afghanistan or Vietnam. Rubies from these two sources have similar characteristic properties.
Photo by Robert Weldon/GIA; courtesy of Edward Boehm/RareSource.

10. Emerald is the bluish green to green variety of
beryl, a mineral species that includes
aquamarine
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.”
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
EMERALD

11. Figure. Emerald speci- men from Chivor, Colombia. The carbon- ate matrix presents a
vug with emerald, and pyrite. The top emerald measures 16.82 × 8.55 mm, the bottom
emer- ald 11.98 × 7.96 mm. The specimen measures 76.43 × 63.44 mm.
Photo by Robert Wel- don/GIA; courtesy of Greg Turner, Corner- stone Minerals.
Figure 60. This ring contains a 12.09 ct sugarloaf cabochon
emerald. Photo by Robert Wel- don/GIA; courtesy of Pioneer Gems.

12. Garnets are a set of closely related minerals
forming a group, with gemstones in almost
every color
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.
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
GARNET

13. All garnets have essentially the same crystal structure, but
they vary in chemical composition. There are more than
twenty garnet categories, called species, but only five are
commercially important as gems. Those five are pyrope,
almandine (also called almandite), spessartine, grossular
(grossularite), and andradite. A sixth, uvarovite, is a green
garnet that usually occurs as crystals too small to cut. It’s
sometimes set as clusters in jewelry. Many garnets are
chemical mixtures of two or more garnet species.
Garnets are a set of closely related minerals that form a
group, resulting in gemstones in almost every color. 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
someblues.
Red garnet is one of the most common and widespread of
gems, found in metamorphic rocks (which are rocks altered
by heat and pressure) on every continent. But not all garnets
are as abundant as the red ones. A green garnet, tsavorite,
also occurs in metamorphic rocks, but it’s rarer because it
needs unusual rock chemistries and special conditions to
form
Demantoid is a rare and famous green garnet, spessartine
(also called spessartite) is an orange garnet, and rhodolite is
a beautiful purple-red garnet. Garnets can even exhibit the
color-change phenomenon similar to the rare gemstone
alexandrite.

14. The essence of the color purple, amethyst is
beautiful enough for crown jewels yet affordable
enough for class rings
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.
Purple variety of the mineral quartz, often form
large, six-sided crystals. Fine velvety-colored
gems come from African mines. In deman for
jewelry at all price point
Mineral: Quartz
Chemistry: SiO2
Color: Purple
Refractive Index: 1.544 to 1.553
Birefringence: 0.009
Specific Gravity: 2.66
Mohs Hardness: 7
AMETHYST