Early examples of the symbol system are found in an Early Harappan and Indus civilisation context, dated to possibly as early as the 35th century BCE.[20][21] In the Mature Harappan period, from about 2600 BCE to 1900 BCE, strings of Indus signs are commonly found on flat, rectangular stamp seals as well as many other objects including tools, tablets, ornaments and pottery. The signs were written in many ways, including carving, chiseling, painting and embossing, on objects made of many different materials, such as soapstone, bone, shell, terracotta, sandstone, copper, silver and gold.[22] Often, animals such as bulls, elephants, rhinoceros, water buffaloes and the mythical unicorn accompanied the text on seals to help the illiterate identify the origin of a particular seal.[23]