You Probably have a slate sink similar to the chemistry lab countertops and sinks in schools. Newer installations use a slate composite which is tougher and easier to maintain.
Per International Stone Works:
Slate can look polished or textured (cleft). It comes in a variety of colors such as gray, green, dark yellow, and even red. Slate tends to be more chemically resistant than some other stones such as marble. Some slate can also be more scratch resistant than other natural stones as well.
They quote and link to the Marble Institute of America, “Stone Dimensions” publication:
(the provided link is 404 - not found, however)
Slate is primarily composed of quartz and illite, with mica, calcite
and other materials. The grey and black colors are due to carbon
and/or graphite. Red shale is caused by the presence of Hematite,
while greens colors are due to Chlorite. Shades of red, purple and tan
result from the amount of iron oxides contained in the stone.
How do you keep slate clean?
Even though slate does not react to chemicals as easily as marble, we still advise cleaning slate with neutral cleaners. We recommend cleaning slate floors with Lavenet neutral stone cleanerand slate tops and showers with Akemi Crystal Clean. Crystal Clean will also help reduce calcium deposits and soap scum build up as well.
We do not recommend power washing or steam cleaning slate floors, especially textured slate. These harsh processes may remove some of the top layers of this foliated natural stone.
We also suggest that customers seal slate on an annual basis to prevent substances such as oil and grease from staining the slate surface. We recommend using Seal & Go® Sfor those who like a natural, no shine look. Alternatively, you can enhance the color of slate to give it the “wet look” by protecting it with Seal & Go® Enhancer S: this sealer will slightly darken the slate without adding shine.