Heliodor is the yellow to greenish-yellow variety of beryl, and the most useful thing to say about it is how many familiar gems are the same mineral.
Beryl is beryllium aluminium silicate. Pure, it is colourless. Trace chromium or vanadium makes it green and it is called emerald. Iron in one oxidation state makes it blue and it is aquamarine; iron in another makes it yellow and it is heliodor. Manganese makes it pink and it is morganite. With no colouring element at all it is goshenite. Five gem names, five prices, one species, and the difference between them is a fraction of a percent of impurity.
Heliodor takes its name from Greek roots meaning gift of the sun, and the name was coined in the early twentieth century for material from what was then German South West Africa, now Namibia. It is therefore a modern trade name attached to a stone that had no separate identity before, which is worth saying plainly, because the name's Greek form invites the assumption of antiquity.
Beryl crystallises in the hexagonal system and forms long prisms, sometimes very large ones, in granitic pegmatites. Crystals of several metres are recorded. The mineral is also the principal ore of beryllium.
Where a classical or medieval source names beryllus, it is generally not referring to this stone or to any single one. As with topaz and sapphire, the ancient term describes an appearance, and matching it to a modern species is a judgement rather than a translation.