Guided Exploration: Gems
Overview: An array of precious and ornamental stones — some uncut, polished, or in elaborate settings — is on exhibit in this hall. Drawn from the Museum’s collection of more than 100,000 minerals and gems, specimens are organized by mineral group and include precious metals. Several cases feature decorative objects and jewelry, which span three millennia and many cultures.
Tip: Bring small flashlights so you can see how specimens react to beams of light.
1. Gem pocket: Minerals are the building blocks of rocks, and form as rocks form. Mineral crystals are typically small, but sometimes grow to large sizes. This is a recreation of a natural, crystal-filled cavity that was found in the mountains of California. Can you find all seven examples of large mineral crystals? (tourmaline, albite, quartz, kunzite, morganite, lepidolite, microcline)
2. Sapphires: A few minerals are used as gems in their natural crystal form. Most, however, are shaped and polished to bring out their sparkle, brilliant color, and/or unusual texture. Gems that are transparent, like these sapphires, are normally faceted: cut with a machine that polishes small, flat windows (called facets) at regular intervals and exact angles. This maximizes light reflected by the stone, highlighting its optical properties and causing it to sparkle. Observe the visual effects of different types and numbers of facets.
3. Star of India: Stones that are not completely transparent are usually shaped and polished into cabochons: a smooth dome shape. This shows off the stone’s color or surface properties, as with opals and star sapphires like this famous 563-carat stone. The ball-like Star of India has stars on both sides. This pattern is caused by tiny fibers of the mineral rutile, which reflect incoming light in a three-dimensional pattern called asterism and also give the gem its milky quality.
4. Quartz: Some minerals, like garnets and diamonds, have the same name as their gemstones. In other cases, the same mineral can give rise to different gemstones because its color varies. So purple quartz is called amethyst, and yellow quartz is called citrine. Look at the way different quartz specimens vary in color and clarity, and compare uncut (natural) specimens to cut ones.
Bonus: Locate the February and November birthstones in this case. Then, find the others around the hall and to note which minerals (in parentheses) give rise to multiple gems.
- January: garnet (garnet)
- February: amethyst (quartz)
- March: aquamarine (beryl)
- April: diamond (diamond)
- May: emerald (beryl)
- June: moonstone (feldspar)
- July: ruby (corundum)
- August: peridot (olivine)
- September: sapphire (corundum)
- October: tourmaline (tourmaline)
- November: citrine (quartz)
- December: topaz (topaz)
What Makes a Gem?
Gemologists use certain properties, some listed below, to determine the quality of a gem.
- Color: depth (not too dark or pale), uniformity, fluorescence
- Clarity: transparent, translucent, opaque
- Hardness: resists scratching (7+ on Mohs scale)
- Durability: will not shatter, crack, or cleave
- Brilliance: high index of refraction, high luster
- Special optics: iridescence (play of colors), cat’s eye or star effects
Did you know? Carat karat. (Or carrot.)
Carat is the standard unit of weight (mass) for gems — not to be confused with karat, the unit of measurement of gold purity. However, both are based on the carob seed, an ancient measure of weight.