This Japanese Ko-Imari blue-and-white Sometsuke dish or plate is made of porcelain.  It was made in the mid 1700's - mid 18th century I believe for other examples I have found with similar pattern and quality.   According to the previous owner who is a Japanese antiques dealer, he dated it as being at least 200 years old or older.

This is an old genuine piece of fine quality.  Used.  No chips, cracks or hairlines.  There is a bit of natural crazing from time of firing (KAN-NYU) in one area on the outer rim - see image.   The few crazing lines cover I would say about a quarter of the outer rim area. 

Width 7.1" : Height 1.2" : Weight 350g

Thanks for looking.