Tosa-suzuri inkstone is a traditional craft originating from Mihara Village in Kochi Prefecture, Japan. It is made using the slates that occurred from the sedimentary succession of the middle Eocene-early Miocene Kurusuno Formation, Hata Group. Traditionally, these slates have been recognized for their “fine texture” that is suitable to make high-quality inkstone. However, such descriptions are based on subjective expressions by calligraphers and inkstone artisans, lacking quantitative and qualitative information. This study employed mineralogical and grain-size analyses primarily utilizing mineral liberation analysis (MLA) to slate samples obtained from past and present quarries for the Tosa-suzuri inkstone production. As a result, the MLA mineral analysis revealed the Tosa-suzuri inkstone consists mainly of silicate minerals. However, many of these minerals could not be necessarily identified, indicating the need for improved mineral detection sensitivity when applying MLA to slate analyses. On the other hand, MLA grain-size analyses revealed that the “fine texture” of Tosa-suzuri inkstone slate is composed of mainly very fine silt. Additionally, through a comprehensive evaluation incorporating the mineralogical and grain-size results, we could identify the inkstone slates obtained from the current Gendani Quarry exhibit higher proportion of sand grains of silicate minerals compared to the slates from past quarries. This result provides data that represents the experienced characteristics of the slates during crafting inkstones by artisans, such as a mention of the “hit a large grain while carving the slate using a chisel” that appears only in the current Gendani Quarry’s slate.