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schiller
see adularescence

scissors cut
mostly found in emerald cut stones, shows crossed, scissor-like crown facets. often encountered in cheap (verrneuil) synthetics.

scratch test
to determine the mohs hardness of minerals, hardly used in gemmology which has to work non-destructively

seal stone
mostly oval or cushion shaped stone with slightly bevelled edge, used for engravings (mongrams, coats of arms etc.). table can be flat or very slightly domed.

semi-translucent
transmitting some light only at thin edges

shapes of cut
the most popular shapes:

single refraction
contrary to doubly refractive materials, singly refractive minerals do not split lightrays in two upon entry

specific gravity
density; the weight of a substance compared to the weight of an equal volume of water at 4°C

square
a square shaped stone, either step- or princess cut

star effect
see asterism

step cut
style of cutting rectangular or octagonal stones with parallel facets arranged like steps of a staircase. the classic octagonal emerald cut is a step cut.

synthetic gems
mostly have the same chemical, physical and optical properties as their natural counterparts. as an example for an exception: synthetic spinels have a slightly higher refractive index than natural spinels.

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