Gemwords are similar to runewords, but use gems instead.

To make a gemword, use a grey (nonmagical, socketed) item and put jewels into all sockets except the last. Then put a gem into it.

The resulting item will have the gem and jewel bonuses, as expected, with one difference: the gem bonus is multiplied by [number of sockets + 1].

Grey long war bow (4 sockets) with 3 jewels + 1 flawless ruby
The item gets the bonuses from the jewels + the flawless ruby bonus x5

This also works in items with a single socket, granting you the gem bonus multiplied by 2.

Gemwords are an affordable low level option for characters that need a specific bonus, whether it be resistances, damage, attributes, etc. without sacrificing a lot of power to obtain them.