Lump chalk is a fantastic natural source of calcium for tortoises. 
Their natural instinct to gain extra minerals is to eat small stones and substrates so this is a much better and safer way. 
It is a completely natural, unprocessed product with no upper limit to feeding. Safe to be left in their indoor or outdoor enclosures at all times for the tortoises to eat as they please. 
Nesting and breeding females will also make very good use of it.
Being natural, the shape, colour and size of pieces will vary considerably.