Gem adjectives are listed in this post. Each word below can often be found in front of the noun gem in the same sentence. This reference page can help answer the question what are some adjectives commonly used for describing GEM.
absolute, antique, architectural, beautiful, best, blue, bright, brightest
brilliant, costly, cut, exquisite, fairest, favorite, first, great
green, hidden, known, literary, little, lovely, new, only
other, particular, perfect, polished, precious, priceless, pure, radiant
rare, real, red, rich, richest, single, small, sparkling
tiny, true, valuable, very
Hope this word list had the adjective used with gem you were looking for. Additional describing words / adjectives that describe / adjectives of various nouns can be found in the other pages on this website.