Meaning of STL?

I am probably being dense but can anyone tell me the meaning of STL? It is on a certificate I have for a stone.
Oh and I have some very nice African sapphire, ruby, garnet, amethyst and emerald available (cut in Sri lanka not native cut).

a copy of the certificate may help to answer your question on STL Chris??

I believe it’s Semi Translucent

Thank you, You are quite correct. After a lot of research i finally found it! It seems to be a lesser but becoming a more popular abbreviation.

Hello Chris!
I stay in Vienna, Austria. I am interested in untreated natural best Stones: Sapphires, colombian
Emeralds and if possible Burmese rubis, all VVS or better. If you want make an offer, pls do it. , Kurt Zalka

Best wishes,