Mishri (in Hindi मिश्री), a form of sugar, is a type of confectionery mineral in the form of large crystallized sugar lumps, generally clear and unflavoured, but sometimes tinted. It has its origins in India and Persia.
Regarded by some as a less harmful than table-sugar, mishri or rock-sugar, which is not as harshly sweet as refined sugar crystals, is sometimes used as a sweetener in beverages (like tea, lemonade) and sweets (like mishri peda and mishri mava). It is also regarded traditionally as having medicinal properties in India and China, and used in some home-remedies including especially those for cough.
Rock candy is also made using Palm Sugar (see here and here), which is considered much superior to the cane sugar version, particularly in medicinal use.
Like the Elaichi Dana, mishri (which is also available as even-sized crystals) is also offered to deity as Bhog and distributed as Prasad.
It is also enjoyed just as a good old-fashioned candy by itself or with rice flakes, roasted bengal gram etc. during harvest time, when sugar is sometimes cooked and poured into moulds.
See also post on Sugar.
Here is a site that gives the recipe to make rock candy as an educational exercise for children (though if intending to eat the result, make sure to use something hygienic as a weight!)