The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Is a Variety of||Feldspar|
These feldspars are rarely encountered in gem form. Their occurrence is widespread throughout the world, in a great variety of rock types and environments, but in most cases transparent crystals are rare.
In many cases faceted gems are identified as a feldspar in the plagioclase series, but the finder does not have the instrumentation needed to pin down the species. This is accomplished by a combination of optical and X-ray analysis. A few plagioclase gems have been well characterized, however, and reported in the literature.
Bytown, Canada, gave its name to bytownite.
Bytownite is found in basic plutonic rocks, some metamorphic rocks, and meteorites. Localities include Montana; South Dakota; Oklahoma; Minnesota; Wisconsin; Scotland; England; Sweden; Japan; and South Africa. Bytownite is sometimes reddish in color and pebbles from Arizona and New Mexico have been faceted into small gems. Bytownite is also reported from Plush, Oregon, but this is a well-known locality for labradorite in facetable crystals; it may be that some of the feldspar has a borderline composition and crosses over into the bytownite range.