The Straight Limestone Column is a beige-white building-block with grey parallel rills on four sides and grey dots around the rims of two sides. It can be crafted in your Crafting Menu (default key "Q") or at Crafting Tables (that still exist in worlds created before R22 on September 16th 2015), but only after the crafting-recipe has been unlocked upon taking a (block of) Limestone for the first time, by mining it from the Fossil layer, which requires at least an Stone Mining Cell. To craft 8 (blocks of) Straight Limestone Columns at a time, you'll need (as of R29 in February 2016):