New England contains many gems both precious and semi-precious. Some of these gemstone deposits are worth millions providing work for many.
Did you know that New England contains some of the most important gemstone deposits in the world? Every state in New England produces gemstones in commercial quantities of one type or another. It is possible to make a living out of New England's gems and some people do. This is evidenced by the number of rock and mineral shops there scattered all over New England. Some of them are part time operations yet others are operated full-time. Many others just operate at gem shows, or over the Internet. All of these shops have one thing in common the close proximity to local gem deposits.
Many of these gems are found in a special kind of rock called a pegmatite. A pegmatite chemically is the same as granite. It forms as a tabular intrusion into other rocks, and by cooling much slower then granite it has giant crystals. The largest single crystal ever found is in a pegamite in the Ural Mountains in Russia
Among the pegmatites the ones that are termed "dirty" containing lithium minerals are most likely to produce gems. The most common is beryl that makes a series of gems starting with the emerald, aquamarine, heliodor and many other colors. Another gem associated with pegmatites is topaz. Even large garnets are seen in pegmatites.
In New England one finds a belt of pegmatites that reaches all the way from western Maine across New England to the eastern side of the Connecticut River all the way to Long Island Sound. This is called the "Bronson Hill Anticlinorium." Throughout the length of this geological feature that covers hundreds of miles one can find gems associated with the pegmatites.
Another source of gems are the trap ridges found along the lower Connecticut River, and in Maine where the most common gem material is Amethyst, and minerals of the Zeolite family. Some of the amethysts found in rocks of the same era along the west side of the Bay of Fundy are in the crown jewels of France.
Much of New England is covered by metamorphic rocks. Here is another rich terrain to collect gems. The most abundant gem associated with these rocks is the garnet. Others are those called anhydrous aluminum silicate. These can take several forms; staurolite- chiastolite, kyanite and sillimanite. Staurolite crystals twin into a cross shaped stone called chiastolite or fairy crosses. Kyanite can be cut into a beautiful blue gem. Sillimanite is also known as fibrolite in this form it is a highly attractive gem.
Garnet / Wikimedia public domain
As it turns out the most valuable gems; ruby, sapphire and emerald are all found in metamorphic rocks. These occurrences are the result of a "dirty" limestone being metamorphosed into marble or amphibolite. The stones are included within the rocks like raisins in a cake.
Even the sedimentary rocks have their own set of gemstones. Usually these stones are some or another member of the quartz family. Agate is common in rocks of this type as well as jasper and bloodstone. In New England the places most likely to have sedimentary rocks are the Connecticut Valley, the Boston Basin from Boston, MA to Newport, RI. Central Maine also contains extensive deposits of sedimentary rocks.
If you don't want to collect your gems from hard rock you might want to go the alluvial route. Gems are always being deposited in the soil by weathering. Eventually they reach the streams and rivers where they are carried as a part of gravel load. This is how most of the colored gems are found commercially. It has the further advantage that the gems found are usually better then those mined from hard rock. The action of the streams breaks up the weaker, less solid stones. In alluvial mining the closer you can get to bedrock the more apt you are to find gems. In this it is like the panning for gold. A gold pan is an excellent device for finding gems too.
What is true for New England is also true of the rest of the World; gems are rare, but they are everywhere. Many gems are found in stream gravel derived from cordierite gneiss. This is the same bedrock found in Sri Lanka and Madagascar. Cordierite is a magnesium, iron, aluminum cyclosilicate found in metamorphosed pelitic rocks. Alone it is also known iolite or water sapphire a gem in its own right. In gneiss it is often found associated with other gems.
If you like to read about gems go here
Cordierite, Wikipedia, http://en.wikipedia.org/wiki/Cordierite
New England Pegmatites, Bob Jones, Rock & Gem, http://www.rockhounds.com/rockgem/articles/pegmatites.html
New England Gems and Gem Mining, John Carter, Associated Content