The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.

Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.

Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.

The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.

In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.

114. The truth of faith was also meant and represented in the Word precious stones, for example, those in the breast plate of judgement, and upon the shoulder-pieces of Aaron's ephod. in the breastplate, the gold, blue, purple, double-dyed scarlet, and fine-twined linen represented matters of love, while the precious stones represented matters of faith deriving from love, as did the two stones of remembrance on the shoulder-pieces of the ephod which were made from shoham encompassed with settings of gold, Exodus 28:9-22. The same point, that the truth of faith is meant and represented by precious stones, is plainly stated in Ezekiel when the subject is the person who possesses the heavenly riches of wisdom and intelligence,

Full of wisdom and perfect in beauty, you were in Eden, the garden of God. Every precious stone was your covering ruby, topaz, diamond, tarshish, shoham, and jasper, sapphire, chrysoprase, and emerald. And gold, the work of your drums and of your pipes, was within you. On the day that you were created they were prepared. You were perfect in your ways from the day you were created. Ezekiel 28:12-13, 15.

Anyone may see that celestial and spiritual things of faith, and not just stones, are meant. Indeed each stone represented a particular essential element of faith.