Corundum show moderate relief, colorless, no cleavage,  and usually with multiple twinning. Importantly it is uniaxial negative mineral

Here is a photo Corundum (C) crystal enclosed in spinel (Sp), alkali-syenite pegmatite (photo 1) and possibly from metamorphic deposit on photo 2, it may give you a general idea.

Colourless (sometimes spotted in blue-sapphire or red-ruby), often hexagonal outline, rather high relief, low birefringence (slightly higher than that of quartz - direct comparison impossible because they do not coexist) imperfect cleavage (partings, frequent), lamelar twins. In the pictures below it is associated with biotite, garnet, sillimanite allowing comparison of the optical properties.

Corundum , if cut perpendicular to 'c' axis shows a hexagonal section. Otherwise the prominent partings perpendicular to the 'c'axis is clearly visible. Crystals are often barrel shaped and can be identified easily. The sections showed in your question has many corundum crystals.

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