Faceted and Cabochons
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
The most important factor in grading sugilite is color. The deepest color receives the highest grade. A mottled appearance under close inspection doesn’t affect the overall value, provided the overall color remains deep. However, lighter areas mixed with dark purple will lower the value.
Sugilite is available in fairly large sizes. Gems over ten carats are common and receive no additional value per carat.
Translucent sugilite isn’t yet available in sufficient quantities for market values to be established.