Gamut Music offers a few options for strings:
Knotted Ends: All strings require a knot at the end to hold them onto the instrument's tailpiece. Many players like to tie their own knots, but Gamut Music will tie the knot for you as an additional service if you want. (Wound strings come with knotted ends by default.)
String Varnish: The varnish offers a protective layer between the surface of the gut and the environment around it. This layer inhibits — but does not stop — the absorption of moisture, oil and acid into the string.
Leather Washers: Leather washers help to keep the string knot from marking an indentation on the underside of a historical tailpiece, or when used with a modern tailpiece, they help to keep the string knot from pulling through the string slot.
What Are Those Little Black Squares That Came With My String??: Those little black squares are anti-tarnish strips put in the envelopes of strings wound in sterling silver to avoid tarnish. Sterling tarnishes, turning black-ish in color, and cosmetically looks bad. The anti-tarnish strips react to hydrogen sulfide and other tarnish-causing gases to neutralize them. If the silver string tarnishes it is still perfectly fine, but these little squares help prevent tarnish.