Sacrifice Lion’s Eye Diamond, Discard your hand: Add three mana of any one color to your mana pool. Activate this ability only any time you could cast an instant.
So the main difference between the printed version of the Diamond and its current official text is that line at the end that says you can only activate the ability any time you could cast an instant. What does that mean?
Normally, if you’re announcing a spell, one of the steps of casting a spell lets you produce mana to cast the spell. So if the Diamond still worked as it was originally printed, you could start casting a spell (card at random: Pox). The first step of announcing a spell is moving it from where you’re casting it from to the stack. Eventually, you’ll get to the step where you can play mana abilities to add mana to your mana pool. You could then tap the Diamond for mana, discarding the rest of your hand, and then finish casting Pox with that mana.
However, that is not how the Diamond was originally designed. When it was originally printed back in Mirage, you had to have all of the mana in your mana pool before you could start casting the spell; so you couldn’t sac the Diamond for mana and then use that mana to cast a spell from your hand (since you just discarded your hand). So that extra line of Oracle text restores the Diamond to its original printed functionality (otherwise, it would be entirely too similar to the much more powerful Black Lotus).
So why do some decks play the Diamond, if you can’t really use it to cast any spells in your hand? Well, one strategy involves using Infernal Tutor. Cast the Tutor using other mana sources, and then, once you’ve cast the spell, sac the Diamond for mana. You’ll get three mana, and since you now have 0 cards in hand, you’ll be able to search for any card in your library when Infernal Tutor resolves, and you’ll have the mana from the Diamond to help cast that spell.
IMPORTANT NOTE: You need to sac the Diamond after you cast the Tutor but before you pass priority to your opponent. This requires you to indicate that you’re retaining priority (e.g., “Cast Infernal Tutor then activate LED”) due to the default tournament shortcut that says you automatically pass priority after casting a spell unless you say otherwise. If you cast the Tutor and then pass priority to your opponent (e.g., by asking if they have any responses), and they pass priority back without doing anything, then the Tutor will resolve and you won’t get the opportunity to sac the Diamond for mana until after the Tutor is done resolving.
One other weird quirk with the Diamond: even though it has a timing restriction of when you can activate the ability, it’s still considered a mana ability. So something like a Pithing Needle won’t stop you from activating the ability since the Needle specifically doesn’t stop mana abilities; and you could do things like activate LED in response to a spell with split second such as Krosan Grip.
Today’s Rules Tip written by Nathan Long