Rock sugar is often manufactured from beet sugar

In Northern Germany, sugar crystals similar to the style depicted on the linked web page are used for "East Frisian Tea".

East Frisian Tea is more a style of tea service than a specific variety of tea, but usually some blend of Indian black teas is used, often Assam and Darjeeling, but some variation from that is possible; the main thing is that it's not an aromatized or fruity tea, but a black tea blend, and tends to be infused with the intention of being slightly bitter.

When serving, rock sugar is placed at the bottom of the cup. Tea is poured over the rock sugar. Then, typically, but not always, a bit of cream is poured on top of that. You don't stir what's in your cup, so you start with a creamy flavor, progress toward a fairly bitter/astringent black tea, and have an increasingly sweet finish. You keep on repeating the process of adding sugar, tea and cream and drinking until you've run out of things to talk about with your friends.

The slow melting process is the main desirable attribute of this type of sugar, not the flavor difference. The brown crystals, with a slight molasses hint, were the most common ones I saw in Germany, but clear ones were not unheard of either. The hint of molasses is the only flavor difference from the brown crystals.

There is also clear ("white") rock sugar (sometimes also known as rock candy or crystal sugar)

https://cooking.stackexchange.com/questions/20367/what-difference-would-using-rock-sugar-make-in-tea