Hornstone

• (n.) A siliceous stone, a variety of quartz, closely resembling flint, but more brittle; -- called also chert.
Found on http://thinkexist.com/dictionary/meaning/hornstone/

Hornstone

Horn'stone` noun (Min.) A siliceous stone, a variety of quartz, closely resembling flint, but more brittle; -- called also chert .
Found on http://www.encyclo.co.uk/webster/H/62

hornstone

alternative name sometimes given to chert; also applied by some to other rocks such as hornfels, which is not treated in this document.
Found on http://www.cst.cmich.edu/users/dietr1rv/gemrxAppB.htm
No exact match found