The six pointed star commonly known as the Star of David has its origins in medieval occultism (both Islamic and Jewish) wherein it was supposed to have been the insignia on King Solomon's ring and contained magical properties.

It's unclear when such beliefs originated, although not necessarily contemporary to Solomon, understanding that the design had some sort of magical property seems to date back as far as the first century AD (Josephus describes the ring being used in the presence of the Roman Emperor Vespaian).

In Arabic, the seal is Khātam Sulaymān and it was believed to have healing and magical properties, including use in alchemy. The seal's motif is present quite a bit of Islamic architecture, especially among those built by rulers and dynasties who placed a premium on astrology (these tended to be the Timurids, Mughals, and Safavids, although not exclusively). (More on this: A. Azfar Moin: The Millennial Sovereign: Sacred Kingship and Sainthood in Islam, 2012) It can be seen in the structures surrounding Humayan's Tomb in Delhi, as well as in some of the structures on the perimeter of the Taj Mahal complex in Agra.

Interestingly, the understanding of the star as the symbol of Judaism is a modern development--it was adopted in 1897 at the World Zionist Congress, interestingly because it was seen as a symbol that could represent Judaism but did not have an overtly religious connotation. See: Handelman, Don; Shamgar-Handelman, Lea (1990). "Shaping Time: The Choice of the National Emblem of Israel". In Emiko Ohnuki-Tierney (ed.). Culture Through Time: Anthropological Approaches. Stanford University Press. pp. 193–226.

For a general overview a, erm, commonly used reference site has a fairly detailed entry under "Seal of Solomon."