Timing is everything. For pickle, mangoes should be fully raw but mature enough to have developed a strong flavour. If they are too tender, they may break down too quickly. If they are too mature, they can lose their sour edge.
In many Indian households, experienced pickle makers often test mangoes by touch and aroma rather than appearance alone. A good pickle mango should feel hard, dense and heavy for its size, with a sharp tangy smell when cut open. The seed inside should still be soft and underdeveloped, which helps the pieces absorb spices and oil more evenly over time.
The best season to buy mangoes for pickle is usually early in the mango season, when raw fruit is abundant and still firm. That is when sellers often have the best stock for achar-making.