You can reroll any Magic or Rare quality Jewels (not Unique) by using the following Horadric Cube recipe.

Sometimes you'll get a high-level Jewel with terrible properties, usually do to very low roll or lower-quality Prefixes/Suffixes, or in the case of a Rare, not enough of them. Since Jewel quality doesn't matter for crafting, you can run this on any difficulty level.

You can usually get at least 1-2 Jewel drops there. Although Jewels are random drops throughout the game, one common strategy for jewel farming is running the Secret Cow Level. Many players farm Jewels to use as components for crafting. Jewels are also an important component in crafting gear in the Horadric Cube. Jewels look like Gems inside a gold setting. Jewels have their own sets of rules and limitations: They come in three qualities: Magic, Rare, and Unique. Jewels are items that can be inserted into socketed armor or weapons that add special new properties to the gear. Classic players, those who have opted not to run with the LoD feature set, do not have access to Jewels. Jewels are only available to players running the Lord of Destruction (LoD) expansion in D2:R.

This guide will cover Jewels, stones you can insert into gear sockets to add properties to armor and weapons. Welcome to Wowhead's guide to Jewels in Diablo II: Resurrected (D2:R).

Diablo 2 lod rune cube recipes