Partner deals and opens 1 club. I have 8 points and 7 diamonds. Opponent has passed. Should I say 3 diamonds to show 7 card suit?
What do you mean 8 points? If you have 8 high-card points and 7 diamonds, your hand is probably too strong for a preemptive jump, especially if most of your points are in your long suit. With a fantastic hand like
♠ 3 2 ♥ 2 ♦ A K J 9 7 5 4 ♣ 4 3 2
you could easily have 3NT opposite a minimum opening such as
♠ A 5 4 ♥ Q 10 4 3 ♦ Q 2 ♣ A 7 6 5
With such a nice hand, your main concern should be whether your partner has major suit stoppers.
In a standard american (or 2/1 game forcing) system, you should start by bidding 1♦, and then letting the auction develop. If your partner rebids 1NT (showing a balanced minimum with stoppers in both majors), you could conceivably raise to 2NT or even 3NT, depending on the type of scoring and your partnership style.
If you have 8 points including distribution, e.g.
♠ 3 2 ♥ 2 ♦ A J 9 7 6 5 3 ♣ 4 3 2
then a preemptive raise is more reasonable, depending on your partnership agreements.
You should better keep that bid (3 diamonds) for splinter (ie club fit and singleton or void in diamonds).
Another question you should answer is: "What does 2d mean?". It should promise 6-card diamonds but: Some play that it's weak (0-5HCP), some others (including myself) play that it's invitational (9-11), some others play that it's strong (13+) and finally some play it as really strong (16+).
If you play that 2d is invitational and your holding is AQJxxxx or AKxxxxx then I would try 2d (especially in IMPs).
The most logic approach however is to bid 1d, and then 2d and then 3d.