Congratultions, Indrek, you have a valuable coin. As you suspect, this is indeed a silver rouble from the old Russian Empire. Most Russian coins of the era have the Byzantine double-headed eagle on the back, but these coins have a stylized cross with crowns pattern.
PETER I ROUBLES: 'I' in the angles of the cross on the back
PETER II ROUBLES: 'II' in the angles
This page applies to both Peter I and Peter II roubles.
The coin in our main picture (upper left) comes from Dr. Christopher Stadler in Bremen, Germany, where it was on sale for $775 euros, about $950 US dollars. The coin is in such good condition, with superb eye appeal, so it can command this amount of money. CoinQuest thanks Dr. Stadler for use of his coin photo. It is a beautiful specimen.
Peter the Great, Peter I, ruled Russia from 1689 to 1725. Then Catherine took over for two years, followed by Peter II until 1730. Catherine's roubles look different than this one, but Peter II minted almost the same coin as Peter I with the cross-and-crown reverse. The values below apply to roubles from Peter I and Peter II. Coins (A), (B), and (C) in the image to the right are from Peter I. Coins (D) and (E) are from Peter II. There is variability in the patterns.
Here are very approximate catalog values. If you have a coin like this, and if it is in good condition, you should seek a knowledgeable collector or dealer to evaluate your coin and recommend next steps, including authentication, grading, and encapsulation by PCGS, NGC, ICG, or ANACS (look them up on the Internet). Our page here gives very general guidance, not absolute facts.
average circulated: $1200
well preserved: $3000
Peter II coins (dated 1727 to 1729) are worth slightly less than this. There are Peter I roubles dated 1724 and 1725 with a special (SPB, St. Petersburg) mint mark that worth over $5000 in average circulated condition. Ask a professional coin dealer or a knowledgeable collector to find the mint mark if you have a 1724 or 1725 rouble.
As with all valuable coins, you must be aware that counterfeits exist. Some counterfeits, especially from well-funded crooks in China, are very difficult to detect, even by experts. Always deal with reputable dealers when there is lots of money involved.
Beware of fakes. Some fakes look very convincing. One 'counterfeit' is not a counterfeit all. It is a dazzling Retro Rouble pattern piece produced as a novelty. They come in various metals and sell for $20 US dollars or so.