This 51st wedding anniversary is often referred to as the "Sapphire Anniversary." The blue sapphire is a symbol of loyalty, trust, and sincerity, which are important qualities in any long-lasting relationship.

Here are some ideas to surprise them:

1. Plan a special dinner: You could plan a surprise dinner for your parents at their favorite restaurant, or even cook a special meal for them at home. You could also decorate the dining area with balloons, flowers, and candles to make it more festive.
2. Plan a surprise trip: If your parents enjoy traveling, you could plan a surprise trip to a place they've always wanted to visit. Make all the arrangements, including flights, accommodation, and activities, and surprise them with the news.
3. Give them a special gift: You could give your parents a thoughtful gift, such as a personalized photo frame or a piece of jewelry that they will cherish. You could also get them a gift certificate to a spa or a concert that they will enjoy.
4. Host a surprise party: If your parents enjoy socializing, you could plan a surprise party for them with close friends and family. Decorate the venue, organize food and drinks, and plan some fun activities for everyone to enjoy.

• Renew your vows
• Recreate your first date
• Take a cooking class together
• Host an outdoor picnic
• Celebrate your top 51 memories
• Do something new together

The 51st wedding anniversary gemstone is Blue Sapphire. It is a testament to the enduring strength and beauty of your love, which has weathered the test of time. So, here's to another year of love, commitment, and cherished memories together!